Mexico; Grey Water Systems - SARAR

Fall
08

Grey Water Systems

Engineers for a Sustainable World
Stanford University

June 8th, 2009

Prepared by:
Jonathan Glassman
Becca Kanegawa
Diane Lee
Andrew Martinez

Final Portfolio

Table of Contents
Table of Contents...........................................................................................2
Executive Summary ........................................................................................3
Assessment of Current Grey Water Systems .........................................................5
Grey Water Content......................................................................................................................5
Water Usage.................................................................................................................................5
System Components .....................................................................................................................5
Sink....................................................................................................................................................5
Grease Trap.........................................................................................................................................6
Filter...................................................................................................................................................7
Terrain..........................................................................................................................................8
Objectives ....................................................................................................9
Grey Water Quality Standards ......................................................................................................9
Discussion of Grease Trap Prototype ...............................................................10
Increased Volume Capacity.........................................................................................................10
Exterior Pipe Connections...........................................................................................................11
Inlet and Outlet Baffles................................................................................................................11
Sludge Catcher............................................................................................................................11
Insights.......................................................................................................................................12
Design Recommendations ..............................................................................12
Sink ..........................................................................................................................................12
Grease Trap ..............................................................................................................................13
Basic Recommendation ......................................................................................................................14
Optional Recommendations ................................................................................................................17
Filter .........................................................................................................................................19
Intermittent Sand Filter......................................................................................................................20
Rapid Multi-media Vertical Filter........................................................................................................23
Proposed Alternatives....................................................................................26
Mulch Basin...............................................................................................................................26
Community Grey Water Treatment System..................................................................................28
Irrigation Application..................................................................................................................28
System Costs................................................................................................29
Next Steps...................................................................................................30
Works Cited.................................................................................................31
Appendix ....................................................................................................33
Filter Treatment Efficiency:.........................................................................................................33

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Executive Summary
Through Engineers for a Sustainable World, Stanford University students collaborated with
SARAR Transformación in an effort toward a sustainable water and sanitation program for
the community in San Miguel Suchixtepec in Oaxaca, Mexico (population: 2,500). The
overall program is broken up into four sub-projects: water quality, grey water systems,
popostero design, and dry sanitation and hygiene. This sub-project concerns the
development of domestic grey water systems.
SARAR Transformación designed the original grey water systems while the World Wildlife
Fund (WWF) and Grupo Saneamiento Ecologico (SAE) installed the systems in 38 homes
in San Miguel Suchixtepec in February 2008. As a result, kitchen and laundry water from
the basin sink was no longer untreated runoff; instead, it was typically diverted through a
particle filter, grease trap, and vertical filter, ultimately supplying water for ornamental
plants. The broader goal of removing nitrates, phosphates, pathogens, and other
constituents through grey water treatment was to reduce the risk of health and
environmental hazards. These hazards included human contamination and eutrophication
in the upper watersheds of the Copalita, Zimatan, and Huatulco river region of Oaxaca,
Mexico. One year later, however, only 8-10 of the 38 grey water systems installed continue
to be properly and regularly maintained.
This report presents design recommendations for an updated grey water system that is more
robust, easily installed and maintained, and made of local materials. The aim through these
design recommendations is to motivate sustained maintenance by the women in the
household who currently take on the responsibilities of the grey water system. Inaccessibility
and difficulty in cleaning factor in to the current lack of grey water system maintenance.
To develop an understanding of both community context and technological considerations,
the team utilized the ESW April 2009 San Miguel Suchixtepec Assessment Report and
“Guía Preliminar” from SARAR Transformación, along with other preliminary materials.
The team continued to conduct an exploration of grey water technologies both in more
developed and developing communities. Based on both cultural and terrain limitations in
the local community and through counsel from SARAR Transformación, the team
ultimately pursued design recommendations to improve the current grey water system in
lieu of proposing alternative technologies or an educational/motivational program.
The design recommendations span over three components of the grey water system: the
basin sink, grease trap, and vertical filter. Influenced by prototyping, existing systems, and
scholarly articles, the recommendations account for appropriate sizing dimensions given
typical domestic water usage flow rates, capacity volumes, and required detention times for
optimal system performance. Along with the dimensional recommendations are the
following suggested design features to promote convenient maintenance.

A colander or strainer-like particle filter is recommended for the sink. These filters come in
many different shapes and sizes to accommodate the non-standard size of sink orifices and

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are available in utility stores. The in-sink filter also removes food scraps and large particles
before the grey water passes through the system. It is also recommended if possible to build
the sink holding table at a height of 65 cm such that the grease trap tank would be able to fit
underneath it.
The grease trap capacity should be approximately 90 L to accommodate for the flow rate of
100 L/hr. PVC Unions or flexible pipe connections can be added to the inlet and outlet pipe
to make the grease trap assembly easier to disconnect for cleaning. A baffle lid design
feature acts to dissipate the inlet stream fluid velocity laterally as well as vertically as to not
disturb the already settled particles or grease layer. A flush drain can be added near the
bottom of the tank for convenient discharge of the contents inside. Optional grease trap
recommendations include a grease siphon, inclined sludge plane, and sludge basket, each of
which serve to minimize direct contact with grey water when cleaning the systems.
The design recommendation for the vertical filter is to implement an intermittent sand filter
in place of the current multi-media filter. While sand filters are used in drinking water
systems and provide for a high quality of effluent, they are also appropriate for these grey
water system because of the possibility of human excreta and pathogens being introduced
into the system. The sand filter is also easier to maintain than the existing multi-media filter
because only the top layer of sand media in the filter has to be regularly replaced, compared
to digging out and replacing all of the particle media in the drum.
Finally, the report includes projected quantitative data including cost and effective effluent
quality calculations based on our design recommendations. Based on the original designs of
SARAR Transformación, these design recommendations for retrofitting and new systems
ultimately contribute to the development of a technically appropriate and contextually
viable grey water system that promotes sustained household maintenance.

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Assessment of Current Grey Water Systems
Grey Water Content
The known content of the community grey water resulting from kitchen and laundry
activities included food particles, cooking oil and grease, various biodegradable soaps and
possible pathogen content from diaper washing (Bulnes, 2009). Other typical constituents
are shown in Table 1.
Table 1: Typical constituents of grey water (Source: “Greywater Reuse in Rural Schools: Guidance Manual”)

Water Usage
Current household water usage included both flow rate as well as capacity data, which was
relevant to the team’s study. Dishes typically produced 20L of grey water in 30 minutes,
while laundry produced approximately 100L in one hour. Overall grey water loads ranged
between 100 – 300 L per day. The intended grey water detention time in the grease trap was
30 minutes (Bulnes 2009). While this data varied among households, it provided a basis for
appropriate sizing requirements.

System Components
All current grey water systems incorporated gravity-flow systems. However, the team
recognized along with SARAR Transformación that the current grease traps and vertical
filters were not sized appropriately for optimal performance. Furthermore, the team
identified components of the grey water system difficult for users to access, operate and
maintain. The effluent water quality of the treated grey water was unknown.

Sink
The current sinks ranged in dimension, with various drainage hole sizes and heights above
the ground. This variation presented challenges for the team to offer standard
recommendations for the sink. The team observed that sufficient height was necessary to
allow proper flow of grey water from the sink. There were no sinks observed that utilized

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any kind of particle filter around the drainage hole. A few homes utilized a consolidated
grey water system design, with the grease trap located directly beneath the sink, given an
accommodating sink height.

Figure 1: Various sink set-ups. (Source: ESW Assessment 2009)

Grease Trap
The current grease trap was not large enough to accommodate the threshold water usage
flow rates and grey water volumes. The current grease trap containers held a volume of 20
L. This capacity was insufficient to handle the flow rate of 100L/hr (Bulnes 2009).
Other design aspects of the grease trap discouraged sustained maintenance due to difficulty
of disassembly from the rest of the system and other tasks that involved a user to be in
frequent contact with grey water.
Through two holes in the grease trap bucket, PVC pipes connected the grease trap to the rest
of the grey water system. These pipes were typically two inches in diameter. They were
located inside the grease trap bucket and consequently often submerged in dirty water,

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grease, and other grime. As a result, they did not allow for easy disassembly and required
user contact with grey water. Due to this inconvenience, grease traps were often idle and
were not emptied out and cleaned regularly.
User contact with grey water during cleaning appeared to be the main factor in discouraging
consistent maintenance. Skimming the top layer of accumulated grease from the grease trap
before emptying the other contents and a submerged particle filter required the user to be in
contact with grey water.

Figure 2: Current grease trap and particle filter designs. (Source: ESW Assessment 2009)

Filter
The last step in the grey water treatment system is the vertical filter. The current vertical
filter systems consisted of a typically 55-gallon drum containing bark, pumice, gravel, and
river stones constituting the filter media. The various media aid in the removal of bacteria,
viruses, turbidity, and other grey water constituents. The removal and treatment efficiency
of the filter is dependent on the media used, the depth of each media layer, and the flow rate
entering the filter.
The effluent from the vertical filter was ideally deposited into a garden for ornamental
plants. While both vertical and horizontal filters were currently used, vertical filters are
more heavily documented in the Assessment Report and are the focus of this report.
In some current filters, influent was disposed onto the top of the filter from a free-flowing
pipe, rather than spread out evenly across the filter in smaller, individual loadings. To solve
this problem, some filters had a perforated pipe placed over the top. While this prevented
an excessive loading on one small area of the filter and thus limited damage due to water
puncturing through the top layer of the filter, the perforated pipe still did not utilize the
entire surface area of the filter.
There were several concerns regarding current media type. There appeared to be a lack of
fine particles included in the filter media to remove bacteria and other constituents in the
grey water. Additionally, there was little information regarding the properties of pine bark
used in the top layer of current filter media. Though the bark was effective because it
fostered the growth of microorganisms that helped treat the grey water, users were unaware
that it had to be replaced frequently. Since the team could not obtain detailed properties of
the media in the current vertical filter, the effectiveness of the effluent was unknown and
could not be calculated.

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The current filter design also discouraged maintenance. As a multi-media filter with larger
media at the top and smaller at the bottom, the fine particles in the grey water got trapped
throughout the entire depth of the filter as the water trickled downwards. This required that
all of the media got cleaned out on a regular basis.
Without regular cleaning, there were not enough pore spaces to allow the water to filter
through and resulting flow rates were drastically reduced. Furthermore, the effectiveness of
the filter decreased with increase use as less of the media surface area was available for the
grey water particles to attach. Many of the households were unaware of the maintenance
requirements of the current multi-media filters.

Figure 3: Current filter designs. (Source: ESW Assessment 2009)

Terrain
Terrain issues were a constraining factor of the current grey water systems. Many of the
households were built on steep and vertical slopes and had extremely limited space for the
grey water system components. As a result, the placement of the current grey water systems
on this terrain hindered accessibility and consequently, convenient maintenance.

Figure 4: Example of local terrain. (Source: ESW Assessment 2009)

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Objectives
The original team objectives were to revisit the current designs of the grease trap and filter,
research solutions to improve efficiency, offer a portfolio of viable system recommendations
and/or technological alternatives, and develop educational/motivational materials. As a
result of later information and counsel from SARAR Transformación, the project focus
shifted solely on improving the current grease trap and vertical filter technology.
The recommendations of this report will focus on the sizing and design features of the sink,
grease trap, and vertical filter. The main focus of the design recommendations is to improve
the ease with which the system can be maintained. The team will also provide data on the
treatment efficiency of the proposed vertical filters, as well as cost calculations for the
proposed system.

Grey Water Quality Standards
There are currently limited standards available for the quality of grey water effluent when
used for landscape purposes. The World Health Organization's only recommendation is that
there is a fecal coliform count of less than 1000CFU/100mL (Table 2). The other guidelines
focus on turbidity, dissolved oxygen, and pH levels. Hence, in the recommendations, the
team has sought to provide removal efficiencies to reduce the fecal coliform count, which
minimizes the potential health hazards of re-using grey water. To best remove any
pathogens, an appropriately designed filter is necessary. Through this filter, fine particles on
the order of micrometers, will be removed from the effluent. This includes bacteria (0.5-10
μm), chemical precipitates (1-1000 μm), and "turbidity" (0.1-10 μm) (CEE 271A Course
Reader).
Table 2: Summary of water quality standards and criteria suitable for domestic water recycling (Salvato, 2003)

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Discussion of Grease Trap Prototype
The team constructed a grease trap prototype using materials that were purchased locally
from Home Depot. The figure below is a visual comparison of the current grease traps used
in the town with the grease trap that was constructed at Stanford.

Figure 5: Current working grease traps in San Miguel
Suchixtepec (Source: ESW Assessment 2009) Figure 6: Prototype developed at Stanford University.

Inlet and Outlet
Baffles

Easy-to-disconnect
connections

Sludge catcher

Figure 7: Grease trap prototype with key features highlighted in red.

Increased Volume Capacity
There are several changes to the original design that have been incorporated in the
prototype. One significant change is the increase in volume. SARAR Transformación has
been aware that the grease traps currently employed in the town are too small for the
expected flow rates. They requested a grease trap with a volume of at least 90 L
(approximately 23 gallons). The container used for the grease trap prototype has a volume
of 18 gallons. Though the prototype volume is smaller than the size suggested by SARAR
Transformación, it was selected because it was a readily available container that was
significantly larger than the original current grease trap volume.

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Exterior Pipe Connections
The team also incorporated features in the prototype to make the grease trap easier to clean,
especially by minimizing direct contact between the user and grey water. One of the
modifications included pipe connections on the exterior of the grease trap container walls
that were easy to remove by twisting. The connections shown in Figure 7 illustrate the
exterior union connections. A flexible pipe fitting with clamps would also work.

Inlet and Outlet Baffles
Inlet and outlet baffles were added to replace the current T-shape pipe connections. The
purpose of the inlet baffle was to direct influent downward so that incoming grey water does
not disturb the grease layer residing on top of the water already within the grease trap. The
outlet baffle prevents grease from exiting with the water effluent. Both of these baffles were
attached to the lid of the container so that they could be easily removed for cleaning
purposes.

Sludge Catcher
The sludge catcher was another feature of the prototype. This design was constructed from
thick metal wires and a metal mesh. Its purpose was to quickly remove accumulated
sediment and sludge at the bottom of the grease trap without requiring the user to
completely drain the grease trap. The catcher utilized wire handles so the user minimized
contact with grey water. The sludge catcher would not eliminate the need to periodically
empty out the contents of the grease trap, but aimed to reduce the frequency of more
rigorous cleanings.

Figure 8: Representation of the process that occurs in the grease trap. Influent enters from the inlet pipe and is
directed beneath the grease layer by the baffle. Within the grease trap that grease is allowed to float to the top while
sludge settles to the bottom. Water is allowed to pass upwards into the outlet pipe toward the filter. The connection
at the outlet pipe prevents grease from exiting with the effluent.

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Insights
Several, but not all, aspects of the prototype were incorporated in the final design
recommendations. While the baffle for the inlet pipe worked well during the testing phase,
that of the outlet pipe was difficult to construct. The inlet pipe baffle was able to direct the
influent downward below the grease layer so that the settled grease layer was not disturbed.
The outlet baffle intended to prevent grease from leaving with the effluent, but for it to work
properly it required adhesion to the side wall of the grease trap so that water only entered
the outlet pipe from below the grease layer. This degree of accuracy was particularly
difficult to achieve in prototyping and was not recommended.

Design Recommendations
Sink
The sink design recommendations do not change the function of the sink but rather work to
easily integrate and improve pre-filtration for the grease trap and vertical filter (Figure 9).

Figure 9: Recommended Sink Configuration

Colander/Strainer Particle Filter
It is recommended that a simple colander or strainer-like filter be used in the sink to remove
larger food and dirt particles (Figure 10). These types of filters will help remove large scraps
to prevent the filter from becoming clogged. Furthermore, with the particle filter located in
the sink, as opposed to inside the grease trap, the user does not have to come in contact with
settled grease trap grey water during maintenance. Strainer-like sink filters come in many
different shapes and sizes and can be found at utility stores or even fabricated from wire
mesh and wire given a skillful hand.

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Figure 10: Prefabricated removable strainers (McMaster Carr, 2009)

Height and Layout
The structure that the sink is on should be sized such that grease trap can be placed
underneath it to utilize space constraints. This means that the height from the bottom of the
sink to the top of the grease trap be about 10 cm. Using the height of the prototype as a
reference, the recommended height of the bottom of the sink to the floor is 50 cm. This is
would put the height of the top of the sink at about 65 cm, which is at about hip level for a
average woman in Mexico (Osuna-Ramirez, 2006). After the sink orifice, the plumbing
pipe should increase to two inches to ensure that the pipes do not clog.

Grease Trap
The recommended grease trap design has many advantages over the existing design and
works to implement simple design components for both existing systems and new models.
The recommendations are broken up into two categories: basic feature recommendations
and optional features (Figures 12 and 19, respectively). The basic features are the easiest and
least expensive to implement in a working system, whereas the optional feature makes the
system initially more expensive and harder to construct, but ultimately promote user
maintenance.

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Figure 11: Grease trap design with representative dimensions shown.

Figure 12: Grease Trap with Basic Features

Basic Recommendation
Container/Holding Tank
The grease trap container should be larger based on design flow rate of 100L/hr and
capacity of 300L/d. For a grease trap to properly function the fluid requires a 30 minute
retention time to settle out the particles and grease. As seen in the table and figure below,
the container should be approximately 90L. This size is based on an empirical fit of current
commercial grease traps (Figure 13) and should be viewed as a fairly conservative number
and allows for a safety factor with regards to inflow rate. An appropriately sized system is

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imperative for proper functioning of the entire system. Current systems are undersized (see
“Current Grey Water Systems” section) which leads to ineffective performance when grease
and particles bypass the grease trap and clog the subsequent filter.

Figure 13: Polynomial fit to existing system volume requirements vs. inflow rate. Data from (Zurn Plumbing Inc.)

Table 3: Recommended grease trap container volume for a given design flow rate

L/hr Fluid Volume Min (L)
20 53.15
40 62.90
60 72.61
80 82.29
100 91.92
120 101.52
140 111.08
160 120.61
180 130.09
Quick Disconnect for Inlet/Outlet pipes
There are two quick disconnection methods, with the first method being advantageous to
the second. The first method consists of a simple flexible pipe fitting with either screw type
or butterfly clamps. The fitting can be found in most utility stores and are widely used in
plumbing applications. The second is a screw type PVC (recommendation for PET) union
(Figure 14). The unions may be more difficult to find in larger sizes but are easy to connect
and disconnect and do not require tools. Both methods allow the user to disconnect the
grease trap container from the plumbing, thus making cleaning the system easier. The user
must no longer reach into the tank to disconnect the pipes, reducing the possibility of
contact with the fluid.

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Figure 14: Inlet and outlet connection recommendations, PVC Union type and flexible pipe coupling type

Bottom Flush Drain
A flush drain can be added near the bottom of the holding tank to make the process of
discharging the holding water during a cleaning cycle easier. The most challenging parts of
installing a bottom drain are ensuring a seal as well as a flush finish on the inside of the
tank. Figure 5 shows one method, which uses PVC bushings and female to pipe connection
to create a force seal between inner and outer surface. This seal can be improved by the use
of a silicon sealer.
The flush drain will only be used when a cleaning cycle is required. The user would situate
the tank so that a smaller more manageable bucket can be placed underneath the drain. This
is accomplished by propping the grease trap up using a brick, wood slap, or even a natural
feature such as a mound of dirt. The user would drain a portion of the tank fluid into the
smaller bucket and then dispose of the bucket into a mulch basin or infiltrating soil. The
system could also be set up to have the flush drain plumbed to an appropriate disposal
location so the user would only have to open the valve to drain most of the fluid out of the
system.

Figure 15: PVC bushing to female to pipe connection

Figure 16: Possible caps for the bottom drain, valve type or screw type

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Baffle Lid
The purpose of the baffle lid is to dissipate the inlet flow velocity horizontally and vertically
to minimize disturbance to the grease layer and settled particles (Figure 17 and 18). The
baffle can be made of a PVC bucket, laterally cut and adhered to the lid. It also allows for
easier access when trying to skim off the grease because the user would not have to navigate
around a T-connection. It should be noted that the T-connection is still needed at the outlet
to prevent the top grease layer from exiting with the effluent (see “Insights” section).

Figure 17: Grease trap top view. Inlet baffle diffuses flow laterally and vertically

Figure 18: Baffle is fixed to the lid for easy removal and to allow for space for the fluid to go under the baffle

Optional Recommendations
The following recommendations are optional to implement as each would add cost to the
system, but would lessen user contact with grey water (Figure 19). These designs should be
prototyped first and integrated on a trial basis before mass implementation.

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Figure 19: Grease trap with all the options

Grease Siphon
The grease siphon design requires grease to be in a liquid state,
which may not necessarily the case. The user would first plug the
inlet and outlet pipes as to isolate the holding tank. Then water
would be added slowly directly to the tank to raise the water
level. The water level would continue to rise as water is added
until it reaches the level of the grease siphon which would then
pull the top liquid grease layer down into the pipe (Figure 20)
and into another bucket that can be carried away for proper
disposal, such as in a mulch basin.

Sediment Collector
The sediment collector design is also optional and could be
implemented (Figure 21). The sediment collector can be made
out of many different materials but some sort of corrosive-
resistant metal or plastic is recommended. The mesh material
Figure 20: Grease siphon in
should be fine enough to trap most sediment but not necessarily
action. Fluid level rises until the all of it. Any sort of mosquito netting should be adequate as it is
top layer is siphoned into the as a particle filter in many applications in the developing
down pipe. world. The intent of the sediment collector is to be able to
quickly clean the grease trap without emptying the tank. This could prolong the time
between intensive cleanings.

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Figure 21: Sediment Collector

Inclined Sludge Plane
The purpose of the inclined sludge plane is to reduce the size of the sediment collector by
focusing the sediment to one area (Figure 22). By concentrating the sediment and sludge to
one area the collection and removal process becomes easier. Care must be taken in installing
this. If the plate is not adhered properly and securely to the tank, then particles may fall
between the plate and sides of the tank. This would cause a large problem and complicate
user-maintenance.

Figure 22: Inclined Sludge Plane

Filter
The design recommendations focus on improving two main concerns with the current filter:
treatment efficiency and ease of maintenance.
The treatment efficiency is based off of the ability to remove particles of different sizes
(mainly in the micrometer scale). This is called the collection efficiency. The removal of
uniformly sized particles in filter is due to diffusion, interception, and sedimentation

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processes. Please see the appendix for equations and parameters chosen for both vertical
filter computations.
The ease of maintenance was qualified according to how often and difficult it is to clean.
The filters recommended require anywhere from weekly to no maintenance.

Intermittent Sand Filter
In order to provide an effective and easy-to-maintain system, the team recommends that an
intermittent sand filter be installed. An intermittent sand filter treats the water to a very high
standard. Unlike rapid (multi-media) filtration, which depends on physical removal of
particles, this bio-filter fosters microorganism growth to absorb and remove pathogens and
other fine colloids. The layer of microorganisms, the “schmutzdecke” or biofilm, forms at
the top few centimeters of the filter and takes two to three weeks to fully develop.
The intermittent sand filter is designed to foster the growth of the biofilm. Like any living
organism, the microorganisms need food, water, and oxygen. There are many rich nutrients
in grey water which serves as food for the microorganisms. To provide adequate water, a
constant water level is required (figure 19). This is why the outlet is 3 cm above the top of
the sand layer. The 3 cm of water is also shallow enough to keep the system aerobic (with
oxygen). The air vent is necessary to keep an adequate flow of oxygen into the system.
The biofilm should not be disturbed. It must be allowed to have sufficient contact time with
the grey water. To do this, an overflow bucket is needed on top of the filter to hold the water
and slowly release it through a 0.05” diameter hole at the bottom (refer to appendix for
calculations). The 5 gal bucket is sufficient to hold enough water for a flow rate up to
300L/d. A diffuser plate, such as an aluminum or plastic sheet with holes in it, will let the
water sprinkle down onto the biofilm. To prevent overloading during the rainy season, a top
lid is necessary to keep the rainwater from entering the system.

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Figure 23: Sketch of critical components of the intermittent sand filter

After the grey water passes through the biofilm layer, it will enter the sand media. The low
filtration rates allow for long detention times in the sand media. The remaining particles in
the grey water are removed in the top portion of the sand layer. Three different media will
be used for the filter (Table 4). Near the outlet pipe, the media may need to be larger than
15cm to prevent clogging.
Table 4: Details of the media used in the intermittent sand filter

Grain
Length
Media Diameter Porosity
(m)
(m)

First layer FINE SAND 3.0E-04 0.8 0.65

COARSE
Second Layer SAND 4.0E-03 0.8 0.05

Third Layer GRAVEL 1.5E-02 0.5 0.05

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Treatment Efficiency
This system will treat the water up to drinking water quality standards, thus meeting the
grey water effluent standards documented earlier. The removal efficiency for two particles,
0.5 and 1.0 μm, is over 99% (Table 5 and Figure 24).
Table 5: Collection and overall removal efficiency for two particle sizes in the intermittent sand filter

Particle Size (μm)
Media
0.5 1.0

First layer FINE SAND 3.89E-03 2.74E-03

Second
Layer COARSE SAND 7.58E-04 7.51E-04

Third Layer GRAVEL 2.26E-04 3.13E-04

Overall Filter Removal
Efficiency (%) 99.9 99.3

Figure 24: Particle removal efficiency for the Intermittent Sand Filter

According to a study done for a flow rate of 0.3m/hr and media size of 0.35mm sand, the
removal efficiencies of fecal and total coliforms, turbidity, and color are all above 88%
(Table 6).

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Table 6: Average percent removal of fecal and total coliform, turbidity, and color given flow rate of 0.3m/h and media
diameter of 0.35mm (Source: (Muhammad, Ellis, Parr, & Smith, 1996))

Average % Removal

FC TC Turbidity Color

98.0 97.3 88.9 88.2

Ease of Maintenance
With regards to maintenance, this system is very
simple to upkeep. Unlike the rapid filters, this
system should only be cleaned when effluent flow
rates become unacceptably low. To clean the filter,
a process called “wet harrowing” should be used.
This is done by first closing the effluent pipe and
removing the lid and diffuser plate. Using a bucket
of clean water, slowly pour the water into the filter.
Being careful not to disturb or touch the biofilm
and sand layers, swirl the water as it is being
Figure 25: Decanting process in a filter (Source:
poured in. This will loosen the dirt and other (Lukacs, 2002))
particles that have accumulated in the filter such
that they will be re-suspended in the water. Then, using a cup, decant the dirt and water
from the filter (Figure 25). If the top of the filter becomes in great disrepair, then the top few
centimeters of sand could be removed. This is not recommended for common maintenance,
as this will destroy the biofilm layer, which takes two to three weeks to grow again.

Rapid Multi-media Vertical Filter
This type of filter is currently implemented in the household’s grey water systems. Unlike
the intermittent sand filter, there is no biofilm layer. To encourage microorganism growth, a
layer of mulch (bark) could be used. The team is recommending six different media layers
that go from a large grain diameter to a smaller one at the bottom (Figure 26 and Table 7).
This will allow the particles in the grey water to get trapped throughout the length of the
filter. The bottom two layers increase in diameter to prevent clogging of the effluent pipe.

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Figure 26: Sketch of the proposed multi-media filter

Table 7: Details of the media used in the multi-media filter.

Grain
Media Diameter Porosity Length (m)
(m)

First layer MULCH 7.5E-02 0.5 0.25

Second
Layer MED. GRAVEL 1.5E-02 0.5 0.10

Third Layer PEA GRAVEL 4.0E-03 0.5 0.10

Fourth Layer FINE SAND 3.0E-04 0.8 0.20

Fifth Layer COARSE SAND 4.0E-03 0.8 0.10

Sixth Layer COARSE GRAVEL 2.0E-02 0.5 0.05

Care should be taken to evenly distribute the flow across the entire top surface area of the
filter. This could be done using the same diffuser plate as described above. A splash plate
could also be used. This is a square piece of durable material that is placed on the top media
layer directly under the inlet pipe. When water hits the splash plate, the water will slowly
fall over the edges of the plate, thus decreasing velocity and spreading it out over a larger
area.

24

A T-connection at the inlet pipe could also help distribute the flow. It should be rotated
often so as not to wear down the same two locations. This last method is not as highly
recommended as the above two.

This filter does not remove pathogens and other small particles as well as the intermittent
sand filter (Table 8 and Figure 27).
Table 8: Collection and overall removal efficiency for two particle sizes in the multi-media filter.

Particle Size (μm)
Media
0.5 1.0

First layer MULCH 6.55E-05 1.31E-04

Second
Layer MED. GRAVEL 1.40E-04 1.78E-04

Third Layer PEA GRAVEL 3.01E-04 2.79E-04

Fourth
Layer FINE SAND 2.52E-03 1.74E-03

Fifth Layer COARSE SAND 4.82E-04 4.47E-04

Sixth Layer COARSE GRAVEL 1.21E-04 1.65E-04

Overall Filter Removal Efficiency
(%) 75.5 62.4

25

Figure 2: Particle removal efficiency for the Multi-media Filter.

Ease of Maintenance
The rapid multi-media filter requires regular maintenance. Since the particles are trapped
throughout the depth of the filter, all of the media will need to be removed and thoroughly
cleaned. This should be done on a monthly basis or when effluent flow rates become
unacceptably slow. The mulch layer will need to be replaced on a weekly basis, depending
on use. If not regularly replaced, the mulch layer will begin to degrade and clog up the filter.
If available, this system could be adapted to be in the form of a dresser or stacked tray
system. Each layer of media would be placed in a separate drawer. This will greatly increase
the ease of maintenance, as it would not be necessary to reach into a large 1m-high filter to
remove all of the media.
The top of the tray system could be open to the atmosphere. This would allow plants to be
grown in the filter. Since components in soap, including sodium, are harmful to most plants,
there is a chance that the plants will not survive.

Proposed Alternatives
The team developed several alternatives to the current grey water treatment system in San
Miguel Suchixtepec. These ideas have been discussed with SARAR Transformación and
are included in this section to illustrate ideas that the team feels may be beneficial to the
town in the future if space and other issues can be resolved.

Mulch Basin
Mulch basins are a low-cost, effective, and easy way of treating grey water. If space
permits, the team highly recommends that a mulch basin be used since it requires the least
amount of maintenance and treats the grey water to the appropriate level. They not only

26

take the place of the filter, but also make the grease trap and particle filter unnecessary. A
major advantage of the mulch basin is that it does not require any extensive engineering,
unlike the other proposed systems. The only engineering that is necessary is the construction
of the pipes and/or trenches that will carry the grey water from the source to the basin itself.
The mulch basin should have an infiltration area of 20m2 (Figure8). This is the area required
for 376L/d (Ludwig, 2006). The pipe could be placed at the same level as the top of the
mulch basin. Grease and particles such as food scraps rest on the top of the mulch and are
allowed to compost. The water is filtered through the mulch material and eventually flows
towards a tree at the center of the basin. If there is concern about having the pipe above
ground, then a flat stone could be placed on top of the pipe to cover it. Also, the pipe could
be placed 6” below the mulch surface and surrounded with stones, creating a small cave.
The only problem with the latter method is that the inlet should be checked on a monthly
basis to make sure that it has not become clogged.

Figure 28: Mulch basin design sketch (Adapted from: (Ludwig, 2006))

A typical home in San Miguel Suchixtepec may require multiple mulch basins. Space can be
limited depending upon where the house is located in the town, thus mulch basins can only
be recommended on a case-by-case basis. Instead of using mulch basins as the primary grey
water treatment option, the team recommends that where space is available, a mulch basin
can be constructed to handle overflow from the grease trap should this be a concern for a
particular grey water system.

There are no quantitative methods available to calculate the treatment efficiency of the
mulch basin. It is known that soil and mulch are excellent natural methods for cleaning grey
water. The living microorganisms in the ground remove most harmful constituents of grey
water and make the water ready for ornamental application.

Ease of Maintenance
This system is by far the easiest to maintain, requiring the user to occasionally remove
particles that may be blocking the pipe and rearranging the mulch material with a shovel. If
a layer of scum forms at the pipe outlet, it can be removed if desired. This is not vital to the
effectiveness of the mulch basin since the water will simply flow over the layer of solidified
scum and infiltrate at another location. If many large food scraps are present in the effluent,

27

then it may be necessary to remove them to ensure that these particles do not block the pipe
opening.

Community Grey Water Treatment System
Another option considered was to develop a centralized community grey water treatment
system. The benefits of a large-scale system are that treatment is consolidated at an off-site
location, thus no longer requiring each resident to deal with cleaning and maintaining
individual grey water systems.
The image below (Figure 29) shows a bird’s eye view of the town. San Miguel Suchixtepec
is situated on top of a mountain ridge. The town can be divided into separate zones where
residences within each zone would have their grey water delivered to a centralized location.
These zones depend on the geography of the town. Since the centralized grey water system
would be entirely gravity-controlled, it would be very difficult and costly to route pipes from
all homes to one centralized location. Instead, multiple treatment locations can be utilized
to service the needs of a particular town area. Grey water can be treated downhill from the
production source and the treated water can then be used to irrigate gardens at the bottom
of the hill and along the slope, or can be released to the environment.

Figure 29: Bird’s eye view of San Miguel Suchixtepec (Google Inc. , 2009)

There are two concerns with this type of system. One concern set forth by SARAR
Transformación is that neighbors may not want to work together to install a collective grey
water system. This type of system requires collaboration from all participants and the fear is
that not everyone will work well together. Another concern is that additional land is
required for this collective grey water treatment system. To obtain this land, the
government must donate subplots for the town to use – this can be a timely and uncertain
process, one in which can be avoided by using individual household grey water treatment
systems.

Irrigation Application
Once the grey water has passed through the entire system, it is ready to be applied to a
household’s garden. Plants that would most likely do well with grey water application are:

28

oleander, rose, rosemary, bermuda grass, honeysuckle, oaks, cottonwood, olive, ice plants,
many native plants, and juniper. Rhododendrons, hydrangeas, azaleas, violets, inpatients,
begonias, ferns, gardenias, camellias, and primroses may also survive, but are more
susceptible to high sodium and chloride levels. Plants that are very susceptible and not
recommended are: crape myrtle, redwoods, star jasmine, holly, and deodar cedar.
(Kennedy, 1995)

System Costs
This section provides an estimate of the costs to construct a grease trap and intermittent
sand filter. The price per piece of material was found in the list of materials provided by
SARAR Transformación. The table below shows the costs for the proposed grease trap.

Table 9: Cost to construct a grease trap

per piece (US Quantity
Material Total Cost
dollars) Needed

Tote (18 gal) $3.88 1 $3.88
Bucket (5 gal) $4.00 1 $4.00
PVC Pipe (20ft) $7.60 1 $7.60
PVC Elbow $0.42 2 $0.84
PVC Tee $0.79 2 $1.58
Flexible Pipe Connection $4.21 2 $8.42
PVC Male Bushing $12.27 1 $12.27
PVC Cap $10.73 1 $10.73
PVC Female Adapt $17.39 1 $17.39
PVC Cement $5.54 1 $5.54
Silicon $3.70 1 $3.70
Wire (16-18 gauge) $4.28 1 $4.28

total $80.23

For the filter, the majority of the cost is due to the container itself (Table 10). Since
households with grey water systems currently installed already have this, the cost is
dramatically reduced.

29

Table 10: Cost to construct an intermittent sand filter.

per piece (US Quantity
Material Total Cost
dollars) Needed

Plastic Tub (200L) $94.40 1 $94.40
Lid for Tub $5.00 2 $10.00
Bucket w/lid (5 gal) $8.00 1 $8.00
Filter Media (sand) (m3) $17.85 0.17 $3.03
Filter Media (gravel) (m3) $11.13 0.03 $0.33
Filter Media (course
$11.13 0.03 $0.33
gravel)
PVC Elbow $0.42 4 $1.68
Connection Pipes $7.60 1 $7.60

total $125.38

Next Steps
Given the proposed design recommendations in this document, SARAR Transformación
can determine if any features should be implemented. The team’s recommendations focused
on providing adequately sized components to handle the threshold capacities, ensuring that
the components would adequately and reliably treat the grey water, and designing the
components to provide convenient, sustained maintenance.
As the Stanford ESW interns leave for San Miguel Suchixtepec for this Summer 2009, the
team hopes that they will be able to build a full-scale working prototype incorporating some
or all of the recommended features. The ten weeks that they are there will provide them
with enough time to use the system and try performing the required maintenance duties.
Since the success of the grey water system greatly depends on household’s willingness and
ability to maintain the system, the interns should conduct a survey with the households that
have the proposed system installed to gauge preference for the new design features. It should
focus on aesthetics and any comments on maintenance duties. As with any project, there are
always ways to further improve systems and make them more appealing. Hence, the survey
will allow future work to focus on the household’s needs and wants.
The water quality team has prepared a GPS system to map out the water system in San
Miguel. If time permits, while the interns map this out, it would be helpful to locate where
all the current grey water systems are. This will help in future decisions in determining if
shared household grey water systems should be considered.

30

Works Cited
Bulnes, M. and Garduño, F. (2009, May 20). Conference Call with SARAR
Transformación.
Ellis, K. (1987). Slow Sand Filtration. WEDC J. Developing World Water , 2, 196-198.

Google Inc. . (2009, May). Google Maps. Retrieved May 2009, from maps.google.com

Grace, P. L. (2009). Whole Filter Models Information Sheet. In P. L. Grace.
"Greywater Reuse in Rural Schools: Guidance Manual.” National Environmental
Engineering Research Institute (NEERI) and UNICEF.
<http://ddws.gov.in/documents/SSHE/Guidance%20Manual%20on%20Greywater
%20Reuse%20in%20Rural%20Schools-English.pdf>.
Huisman, L., & Wood, W. (. (1974). Slow Sand Filtration. Geneva.

Kennedy, D. (1995). Using Greywater in your landscape: Greywater Guide. State of California,
Department of Water Resources.
Ludwig, A. (2006). Create an Oasis with Greywater. Oasis Design.

Lukacs, H. (2002). rom Design to Implementation: Innovative Slow Sand Filtration for Use
in Developing Countries. MSc Thesis. Massachusetts Institute of Technology, USA.

McMaster Carr. (2009, May). McMaster Carr Online Catalogue. Retrieved May 2009, from
www.mcmaster.com
Milena, J. E. (2009). Assessment Trip. Stanford University, Stanford.

Muhammad, N., Ellis, K., Parr, J., & Smith, M. (1996). Optimization of slow sand
filtration. Reaching the unreached: challenges for the 21st century. 22nd WEDC Conference,
(pp. 283-285). New Delhi.
National Environmental Engineering Research Institute (NEERI). (1982). Slow sand
filtration. Final project report, Nagpur, India.

Osuna-Ramirez, I. H.-P. (2006). Indice de masa corporal y percepcion de la imagen
corporal en una poblacion adulta mexicana. Salud Publica de Mexico , 48 (2).

Salvato, J. N. (2003). Environmental Engineering. Hoboken, NJ: John Wiley and Sons, Inc.

SARAR Transformación. (n.d.). Guía Preliminar de Filtros de Aguas Grises.
Van der Ryn, S. (1978). The Toilet Papers: Recycling Waste and Conserving Water. Sausalito,
California: Ecological Design Press.
Water for the World. Constructing a Household Sand Filter, Technical Note RWS. 3.C.1.

31

Yao, K. H. (1971). Water and Waste Water Filtration: Concepts and Applications. Environ.
Sci. Technol. , 5 (11), 1105-1112.

Zurn Plumbing Inc. (n.d.). Grease Traps. Retrieved May 2009, from Zurn Plumbing
Products: http://www.zurn.com/pages/catalog.asp?
ProductGroupID=73&OperationID=7#

32

Appendix
Filter Treatment Efficiency:
The particle removal efficiencies for the intermittent sand filter and rapid multi-media filter,
are based on diffusion, interception, and sedimentation processes (Figure 30). Straining
processes are where particles that are larger than the spaces between the media become
stuck and hence removed from the water.

Figure 30: Basic transport mechanisms in filtration (Source: (Yao, 1971)).

Interception is where the water flow streamlines are close enough to the collector (the media
in the filter) to attach to the media. Sedimentation is due to the gravitational forces and
allows the particles to move across streamlines and attach to the media. Lastly, diffusion is
due to random Brownian movements that brings particles near the media.
Combining these processes allows the computation of the removal efficiency of a uniformly
sized particle. Table 10 shows the parameters used for this analysis.

33

Table 1: Parameters for filter analysis (Adapted from: (Grace, 2009))

According to Yao et al. (1971), the equations for a single media filter are:
Diffusion Contact Efficiency:

Interception Contact Efficiency:

Sedimentation Contact Efficiency:

The sum of the above processes yield the contact efficiency for a single grain:

34

Thus, the collection efficiency for the entire media of a filter is:

Using the following equations, the performance of a packed bed and hence the removal
efficiency is related to the efficiency of a single spherical collector:

For a multiple media filter, the overall ratio of the filter effluent to filter influent is:

The results have been documented in the Filter section.

Acknowledgements:
Models were adapted from Grace, P., Luthy, R. “Whole Filter Models Information Sheet,”
Feb. 2009. Original models from Yao, K.M., Habibian, M.T., O’Melia, C.R.: Water and
Waste Water Filtration: Concepts and Applications. Environ. Sci. Technol., Vol 5, No. 11, pp
1105-1112, 1971, Dept. of Environmental Sciences and Engineering, University of North
Carolina, Chapel Hill, N.C. 27514. Reprinted in CEE 271A Course Reader, section 11-A,
Winter 2009.

35

Mexico; Grey Water Systems - SARAR

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