Total suspended solids (TSS), volatile suspended solids (VSS), and biochemical oxygen demand (BOD) are key parameters used to analyze wastewater and biosolids. TSS is determined by filtering a sample and weighing the solid residue, while VSS is the weight loss when ignited at 550°C. The sludge volume index (SVI) measures sludge settling characteristics important for activated sludge process design and operation. Colloidal solids cause turbidity which is removed through bioflocculation in biological treatment.

1. Suspended Solids
(TSS, VSS, Biodegradable VSS, SVI
and Colloidal solids )

2. Suspended Solids
• Total solids (TS): Material residue left behind after
evaporation of a sample and its subsequent drying in a oven
at a defined temperature to constant weight
• Total dissolved solids (TDS): Portion of the material residue of
a sample that passes through a filter
• Total suspended solids (TDS): Portion of the material residue
of a sample retained by a filter
– Settlable solids: Material settling out within a defined period
• Fixed suspended solids: Residue of TSS left after ignition for a
specified time at a specified temperature
• Volatile suspended solids: weight loss on ignition of total
suspended solids
– Biodegradable volatile suspended solids: volatile suspended
solids lost through biodegradation
• Colloidal solids: cause turbidity and measured as turbidity
(NTU or JTU)

3. Suspended Solids
• Regulatory limits are imposed on TSS for sewage disposal
– Water with high suspended solids may be aesthetically
unsatisfactory (for bathing!)
• Removal of TSS is one of the sewage treatment objectives
– Primary treatment is mainly concerned with it
• Sludge generation calculations in biological treatment require
the knowledge of TSS, VSS and biodegradable VSS
– All VSS is not biodegradable, and biological treatment can
hydrolyze only the biodegradable VSS
• Biological treatment involves generation of suspended solids
(biosolids)
– These biosolids are monitored as MLSS (TSS) and MLVSS (VSS)
• Maintenance of higher levels of biosolids (activated sludge) is
important in biological treatment
– MLVSS is often used as a measure of active biomass/sludge
• SVI used in the design, operation and control of secondary
clarifiers require MLSS (TSS) monitoring

4. Total suspended solids (TSS)
TSS and MLSS are one and the same
Two alternate ways for TSS measurement
• Filter the sample through a weighed ash free filter paper, dry
the filter paper along with the residue retained on it to
constant weight at 103-105C, and gravimetrically find the TSS
– High measurement uncertainty values – in case of low TSS larger
volumes need sampling
– In case of samples with high TDS thoroughly wash the filter
paper with TDS free water to remove the dissolved material
• Find TS and TDS for the sample and take difference of TS and
TDS as TSS
– In case of the filter paper clogging and prolonged duration of
filtration this method is followed
Often settlable solids rather than TSS is measured as an
alternative
• Centrifugation for TSS measurement?

5. Volatile Suspended Solids
• VSS and MLVSS are one and the same
• Weight loss on ignition of the TSS represent the VSS
• Ash free filter paper leaves no residue on ignition
• Negative error is introduced from the loss of volatile matter
during drying
• Estimation of low concentrations of volatile solids in the
presence of high fixed solids concentration can be more
erroneous
• Dried residue left on the ash less filter paper is ignited to
constant weight at 550±50C in a muffle furnace to remove
volatile matter and obtain fixed or non-volatile matter
– Difference of TSS and NVSS (fixed solids) is taken as VSS

6. Solids in Samples with Solids > 20,000 mg/L
The methods used for samples with lower solids levels are not
used – can be associated with negative error
If the sample is a sludge, stir to homogenize and place it in a
evaporation dish, evaporate to dryness on a water bath, and
dry at 103-105C for 1 hour to find % solids
For finding fixed and volatile solids ignite the residue in muffle
furnace for one hour at 550±50C
– If the residue left in the evaporation dish contains large
amounts of organic matter then ignite it first over a gas burner
and then in the muffle furnace
 
BC
BA
solidstotal



1000
%
 
BA
DA
solidsvolatile



1000
%
 
BA
BD
solidsfixed



1000
%
A - weight of dish with residue
B - weight of the dish
C - weight of dish with wet sample
D - weight of dish with residue after ignition

7. Settlable Solids
• Determined on either volume (mL/L) or weight (mg/L) basis
• Measurement on volume basis requires an Imhoff cone
– Fill the cone to 1 L mark with sample and settle for 45 min.
– Gently stir sides of the cone with a rod by spinning and settle for
another 15 minutes
– Record volume of the settled solids in the Imhoff cone
• Measurement on weight basis
– Determine TSS of well mixed sample
– Pour >1-L of sample into a glass vessel of >9 cm dia. to depth
>20cm and let it stand quiescent for one hour
– Without disturbing the settled and floating material siphon out
water from the vessel center and determine TSS as non-
settlable TSS
Settlable solids = TSS – non-settlable TSS

8. Sludge Volume Index (SVI)
• Volume in mL occupied by 1 g of a suspension after 30 min.
settling
• Used to monitor settling characteristics of activated sludge
and other biological suspensions
– Determined for the mixed liquor of the aeration tank of the ASP
• Determine TSS concentration of a well mixed mixed-liquor
sample
• Use Imhoff cone for settling 1 L of well mixed mixed-liquor for
30 min. time and measure the settled sludge volume in mL
– Gently stir the sample during settling
• Calculate SVI as
)/(
1000)/(
Lgionconcentratsolidssuspended
LmLvolumesludgesettled
SVI



9. Colloidal Solids and Turbidity
• Colloidal matter causes turbidity
• Turbidity is an optical property caused by scattering of light,
and indicates clarity of water
• Biological treatment removes colloidal solids/turbidity
through bioflocculation
• Nephelometers are used for measurement and the results are
reported in Nephalometric Turbidity Units, NTU
– Intensity of light scattered by the sample is compared with the
standard reference suspension under the same conditions
• Formazin polymer suspension is used
• A light source and a photoelectric detector are used in the
measurement