This document discusses continuous stirred tank reactors (CSTR) for treating dairy wastewater. CSTRs are completely mixed anaerobic reactors that maximize contact between biomass and waste to optimize digestion. They have low operating costs since they produce biogas and are suitable for high-strength organic wastes like those from dairy processing. CSTRs are simple in design with short construction periods and can effectively treat wastewater from industries like dairies, breweries, and food processing. The document provides details on CSTR configuration, advantages, applications, specifications and inhibition challenges from lipids in dairy wastewater.

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KARNATAKA VETERINARY, ANIMAL & FISHERIES
SCIENCES UNIVERSITY
DAIRY SCIENCE COLLEGE, BENGALURU-24
Department of Dairy Technology
COURSE: Waste disposal and pollution abatement
TOPIC: Continuous stirred tank reactor (CSTR)
Submitted by,
Chandana R, DHK1908
Da-i-hokmitre shylla, DHK1909
Dhananjaya S, DHK1910
Submitted to,
Dr. Thejaswini M L
Assistant professor
Dept. of Dairy Technology
DATE: 27-July-2023

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INTRODUCTION
The dairy industry is one of the most polluting of the food industries. Dairy is one of the major
sources of water pollution. Given the increased milk demand, India's dairy industry is expected
to expand rapidly, with waste generation and related environmental issues becoming
increasingly important. When discharged to the surface land or water, poorly
treated wastewater with high levels of pollutants caused by poor design, operation, or treatment
systems causes major environmental problems.
Dairy waste water contains highly putrescible organic constituents. This necessitates prompt
and adequate waste water treatment before disposal to the environment. Almost all of the
organic components of dairy waste are biodegradable. As a result, the wastewater can be treated
biologically, either aerobically or anaerobically. These wastes are considered potential
pollutants when they have a negative impact on the environment and are typically released in
the form of solids, liquid effluents, and slurries containing a variety of organic and inorganic
chemicals.
EFFLUENT TREATMENT:
Effluent treatment in industries to meet discharge standards has always been a major issue
for industrialists. Every industry effluent treatment plant must treat effluent for this purpose in
their own industry through effluent treatment plants. Before discharging treated effluent onto
land or into any surface water body, industries must comply with the effluent discharge
standard norms. Characterization of waste water, treatability studies, and planning of proper
units and processes for effluent treatment are all required for proper processes in the ETP.
While the industry manufactures milk, butter, or cheese through processes such as
pasteurisation or homogenization, high levels of BOD (Biochemical oxygen demand)
and COD (Chemical oxygen demand) are produced, which must be treated before being
discharged into the environment. Other common effluents include suspended solids, milk fat,
and dairy odours that must be addressed.
Every dairy production unit requires a Dairy Wastewater Treatment Plant that efficiently
addresses unbalanced levels of BOD, COD, suspended and dissolved solids, resulting in safe
industrial waste disposal.

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Operation of dairy industry with effluent treatment:
The dairy effluent treatment plant follows a set of steps:
· The pH is first adjusted to 8.5 using pH controllers such as caustic or acid. Any emulsions are
then broken down and solids precipitate with the help of a de-emulsifier.
· Other important steps in the treatment of dairy effluent include flocculation and dissolved air
flotation. The wastewater is flocculated by passing it through a slow mix zone, where the
particles are gathered together to form larger ones, which are then treated further using the air
flotation technique.
· The dissolved air flotation technique works as follows: air flotation system bubbles are
propelled by a Recycle Air Dissolving system, which blows the treated effluent, pressurises it,
and dissolves it with air.
· Finally, the sludge batch is pumped through the filter press and disposed of in accordance
with environmental regulations.
Finally, the Effluent Treatment System designed specifically for the dairy industry results in
efficient resource use, lower operating costs, smooth dairy functions, compliance with
environmental regulations, and peace of mind.
ANAEROBIC DIGESTOR
Anaerobic treatment is a proven and energy-efficient method for treating industrial wastewater.
It uses anaerobic bacteria (biomass) to convert organic pollutants or COD (chemical oxygen
demand) into biogas in an oxygen-free environment. Anaerobic micro-organisms (specific to
oxygen-free conditions) are selected for their ability to degrade organic matter present
in industrial effluents, converting organic pollutants into biogas (methane + carbon dioxide)
and a small amount of biosolids. The energy-rich biogas can then be used for boiler feed and/or
combined heat and power (CHP) to produce ‘green’ electricity and heat.
Anaerobic treatment offers several advantages over aerobic alternatives:
Low energy use
Small reactor surface area
Lower chemical usage
Reduced sludge-handling costs. Use aerobic treatment after anaerobic treatment to achieve a
good treated-water quality for discharge into a water course.

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CONTINUOUS STIRRED TANK REACTOR
CSTR is completely-mixed form of anaerobic reactor, designed to maximize the contact
between the biomass and the waste, to optimize digestion performance. CSTR digester
configurations, it is also one of the best in terms of applicability to different wastes. The
wastewater (wastewater/sludge mixture) enters the reactor at the bottom and leaves at the top
and includes an internal recycle loop which draws reactor contents from the opposite side of
the entry. Proper mixing by means of a top-entry, central agitator ensures that the influent is in
constant contact with the biomass for optimal mass transfer and conversion of organic content
(COD) to biogas.
Advantage Of CSTR Process:
• Low Operating expenses, produces biogas as by product which can be used as
energy source.
• Based on proven process, does not requires mechanical aeration and less power
consuming.
• Very suitable for high organic wastes.
• The system can handle sludge, slurries or other concentrated wastewaters;

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• Suitable for high fats, oil and grease (FOG) concentrations;
• Can manage very high suspended solids (TSS) concentrations.
• Suitable for high protein (nitrogen) content wastes.
• Simple design short construction period
• Optimal heat- and material distribution inside the digester
• Large digester volumes possible
Applications:
• Petrochemical and field operations.
• Dairy processing (milk, cheese, yogurt)
• Beverage factories (breweries, juice, soda)
• Municipal sewage plants
• Pulp & paper
• Aquaculture & hatcheries
• Food processing, Tanneries animal products wastewater
Specifications & Special Features:
• Available in any capacity, in prefabricate & in civil tank type configuration.
• Optimum design for CSTR process with low power requirement and lower op-
ex.
• Available in many configuration and options in type, Material of construction,
• Adequately designed pre and post treatment
• Easily expandable and suitable for augmentation projects of old treatment plants
to enhance capacity and quality of treated water.
• Suitable technology for phase wise operation & start-up of system.

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H2O CSTR Standard Model Configuration
MODEL CODE CAPACITY TYPE FEATURES
HPCSTR-5000 Suitable upto 5000
Kg/ COD
Fabricated structure
in MS / CS / GFSS
tanks. Anticorrosive
painting or lining
option available.
Suitable option for
export applications.
Rapid construction
time.
HPCSTR- CL Any capacity above
100 Kg/Day COD
Tanks in Civil
construction
Long life durable
option.
Highly loaded wastewater from the dairy industry (COD ≥ 1,000 mg/L) can preferably be
treated by activated sludge processes in combination with flotation and anaerobic pre-
treatment such as expanded granular sludge bed (EGSB) reactors for reduction of energy costs.
Concerning sludge management, the highly energetic flotation sludge and the waste activated
sludge (WAS) of the aerobic post-treatment are generally disposed of as co-substrates to
municipal wastewater treatment plants (WWTP) or biogas plants. Anaerobic treatment of
these energy-rich substrates in digesters on site combined with a combined heat and power unit
(CHP) would offer major benefits, such as the reduction of disposed sludge volumes, an
increase of methane generation up to 50% and an increase of overall energy efficiency in case
of integration of waste heat usage of CHP units in industrial production. However, operation
of anaerobic co-digestion with major shares of flotation sludge (over 80% COD), which
consists notably of lipids, is challenging. Successful examples of lipid-fed digestions have been
reported using various lipid-rich substrates as well as inhibitions related to lipids.
Inhibition is mostly related to accumulation of long chain fatty acids (LCFA) which are an
intermediate product of anaerobic lipid degradation. Angelidaki and Ahring reported threshold

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concentrations for oleate as 0.2 g/L and stearate as 0.5 g/L. New publications assume that
LCFA might build up layers on the biomass that might reduce mass transfer of substrates.
Inhibition on methanogens is consequently based on total solids related specific LCFA
concentration or surface related LCFA concentration. For anaerobic sludge from a treatment
plant treating milk fats, Hwu et al. reported a 50% loss of methanogenic activity at oleate
concentration of the sludge of 32 mg/g TS. Adding lipid-rich synthetic dairy wastewater to
an AnMBR, reported a loss of methanogenic activity up to 50% at LCFA concentrations above
100 mg/g TS although methane yields remained stable. provided a threshold value for LCFA
concentration of the sludge of 100 mg LCFA/g TS. Precipitation induced by divalent cations
can reduce the toxic effects of LCFA at the expense of reduced biodegradability. Additions
of bentonite reduce the toxic effects of LCFA by providing additional surface for adsorption in
the reactor.
Maximum substrate specific volumetric loading rates (OLRs) of FOG and various lipid-rich
substrates have been published as design criterion for co-digestion in CSTRs. summarized FOG
related studies and recommended an average OLR of 0.84 kg VSFOG/(m³∙d) and a maximum of
2.5 kg VSFOG/(m³∙d). Recommended OLRs for sludge digestion vary between 1.9 and 3.2 kg
VS/(m³∙d), 1.6–4.8 kg VS/(m³∙d) and a degradable COD related maximum of 2.9 kg
CODdeg/(m³∙d). Maximum sludge loading rates are rarely published. Observed VSS specific
lipid loading rates ranged between 0.04 and 0.13 g lipids/(gVSS∙d) when treating particulate-
free wastewaters.
Anaerobic membrane bioreactors (AnMBRs) are used to decouple hydraulic and sludge
retention time compared to conventional fill-and-draw CSTRs. This offers the opportunity to
extend degradation by increasing SRT or keeping an applicable sludge load at similar SRT but
reduced reactor volumes. At this writing, only a few studies have been published that apply
AnMBR for mesophilic anaerobic digestion of particulate substrates. Assessment of research
data shows inconsistent results concerning achieved methane yields in comparison to
conventional CSTRs. observed small increases of VS removal and higher methane yields
compared to a CSTR at equal SRT, achieved similar methane yields when feeding sewage
sludges. observed higher OLR and higher degradation rates in AnMBRs fed with lipid-rich
kitchen slurry and related this result to a positive effect on enzymatic hydrolysis activity.
This study deals with the feasibility of anaerobic co-digestion of WAS and flotation sludge
from the dairy industry with COD shares of lipids up to 85%. The influence of SRT and shares
of flotation sludge on methane yield, process stability, and inhibition on acetoclastic
methanogenic activity are determined. Conventional CSTRs and an anaerobic membrane

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bioreactor were operated with HRT and SRT below common design recommendations under
mesophilic conditions to investigate limitations of the loading capacity of anaerobic digesters
rather than maximizing methane yields. As the introduction of additional solids can reduce the
inhibition, the question arises if the joint digestion of flotation sludge with high shares of lipids
and WAS with considerable amounts of non-degradable solids results in significantly lower
inhibiting effects and successful operation with higher sludge loads. Finally, design criteria and
potentials of AnMBR for overcoming lipid-related limitations are elaborated.
2. Material and methods
2.1. Substrate sludge characteristics
WAS from a WWTP with 40,000 PE and flotation sludge originated from ice
cream production were sampled at least weekly. Data on mean compositions of both substrates
WAS had a mean COD concentration around 48.2 g COD/L. COD concentration of the
flotation sludge ranged around 188.3 mg COD/L. Calculated lipid content of the flotation
sludge was above 0.9 g CODli/g CODsubstrate. The lipid content of the WAS is low (~0.2 g
CODli/g CODsubstrate).
Table 1. Characterization of substrates (arithmetic mean ± standard deviation).
Parameters WAS Flotation sludge
value n value n
COD [g/L] 48.2 ± 8.5 34 188.3 ± 44.9 26
Total solids [%] 4.1 ± 0.7 34 6.8 ± 1.4 26
Volatile solids [%] 3.1 ± 0.6 34 6.5 ± 1.4 26
COD/Volatile solids [g/g] 1.6 ± 0.1 – 2.9 ± 0.3 –
Nitrogen content [mg TNb/g CODin] ~25 10 ~10 10
Protein content fpr [gCOD/gCOD]a
~0.25 – <0.1 –

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Parameters WAS Flotation sludge
value n value n
Lipid content fli [gCOD/gCOD]a
~0.2 – >0.9 –
carbohydrate content fch [gCOD/gCOD]a
~0.55 – <0.1 –
n: number of analysis.
a
fractionation of COD was calculated based on COD/VS-ratio and nitrogen content of the
measured substrate and fraction specific theoretical ratios by maintaining a nitrogen, COD and
mass balance according to Poggio et al. [21] using the following formulae and theoretical
specific ratios.
fpr=gNsubstrategCODsubstrate⋅gCODprgpr⋅gprgNpr.
fpr+fli+fch=1.
proteins: 1.25 g COD/g VS and a nitrogen content of 0.137 g N/g VS.
lipids: 2.87 g COD/g VS and a nitrogen content of 0 g N/g VS.
carbohydrates: 1.18 g COD/g VS and a nitrogen content of 0 g N/g VS.
2.2. Reactor setup and experimental procedures
Continuous digestion tests were performed using an AnMBR and a CSTR in pilot scale. A
simplified flow scheme of the reactor setup is given. The AnMBR (EnviroChemie GmbH,
Germany), has a digester volume of 2.1 m³ and is fully automated.
The microfiltration membrane unit consists of rotating ceramic disc filters (KERAFOL,
Germany) with a nominal pore size of 0.2 μm mounted on a hollow shaft (Amembrane ≈ 3.36 m2
).
Speed of rotation ranged from 250 rpm to 350 rpm. The membrane unit allows decoupling of
SRT and HRT. CSTR operations with SRT = 10 d and SRT = 15 d were also conducted in this
reactor, but without decoupling of HRT and SRT. CSTR operations with SRT = 20 d were
conducted in a separate digester with a volume of 1.0 m³. AnMBR and CSTR were fed with
prepared mixtures of WAS and flotation sludge from a common buffer tank.

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Fig. 1. Simplified flow scheme of reactor setup in pilot scale.
FC: Flow control,
FI: Flow indication,
PI: Pressure indication,
QI: pH indication (in digester);
CH4, CO2 indication (in biogas),
TI: Temperature indication,
WC: Weight control.
2.3. Experimental procedures
The CSTRs and the AnMBR were inoculated with digested sludge from a full-scale digester
of a municipal WWTP. Mesophilic conditions with temperatures around 37 ± 1 °C were
maintained. For comparison of both reactor types, CSTRs were operated at SRT = 10 d, 15 d
and 20 d while AnMBR was operated at HRT = 10 d and SRT = 15 d. Both reactors were fed
continuously. The mixture was prepared in regular intervals (3–5 times per week). The
composition of the mixture was changed stepwise after operation of three sludge retention
times. Investigated COD related shares of flotation sludge (CODFS) to total COD load of the

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inflow (CODFS/CODtotal) were 0.5, 0.6–0.7 and 0.8–0.9. OLR are given. After two SRT without
major changes of operation, a steady state was reached. The performance in steady state was
evaluated for another SRT. The first SRT after change of feedstock is defined as transient state
and was investigated as well.
3.2. Operational performance of continuous digestion tests in pilot scale
methane yields and applied sludge loading rates of continuous digestion tests in pilot scale.
Increasing shares of flotation sludge were successfully degraded in all reactors at SRT down
to 10 days. In CSTRs, highest methane yields up to 280 L/kg CODin were achieved at COD
shares of lipids over 80% and SRT = 20 d. At similar COD shares of lipids, slightly lower
methane yields were obtained with applied SRT ≈10 d compared to 20 d. The AnMBR,
operated at HRT = 10 and SRT = 15 d, achieved methane yields higher than 280 L/kg CODin at
COD shares of lipids around 85%. During the entire operational period, methane yields in
relation to degraded COD always ranged between 320 and 350 L/kg CODdeg in all reactors
resulting in a COD based gap of mass balance less than 10%. This indicates that lipids were
available for anaerobic degradation and did not accumulate on the liquid surface in the digester.
As expected, total solids and volatile solids concentration are higher in the AnMBR than in the
CSTRs while their ratio is similar. The organic acid concentrations in the reactor remained
below 1,000 mg/L in all operational settings. The pH value remained between 7.2 and 7.7 in
all settings.
Fig. 2. Methane yields of CSTR and AnMBR with SRT = 15 d compared to maximum methane
yields based on results in batch-scale (dashed line) as a function of COD related shares of
flotation sludge (a); Methane yields of reactors fed with high COD related shares of flotation
sludge > 0.8 as a function of SRT compared to calculated methane yields as a substrate first-
order reaction

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CONCLUSION:
Unlike many other industries, the dairy industry is at the top of the list in terms of industrial
waste production. The dairy industry process reveals that approximately 2 litres of water are
used to process 1 litre of milk. This clearly shows the massive scale of effluents that must be
treated in order to improve operations and comply with environmental regulations.
Continuous stirred tank reactors (CSTR) are widely used in wastewater treatment plants to
reduce the organic matter and microorganism present in sludge by anaerobic digestion. The
characterization of the sludge flow inside the digester tank, the residence time distribution and
the active volume of the reactor under different criteria are determined. The effects of design
and power of the mixing system on the active volume of the CSTR are analyzed.
REFERENCE
• Water Research
Volume 71, 15 March 2015, Pages 282-293
• https://doi.org/10.1016/j.wri.2019.100122
• VEOLIA kingdom of equipments.
• wikipedia