Sewage and its treatment - experience from setting up STPs

Sewage and Its Treatment : Experience from setting up Sewage Treatment Plants

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ABSTRACT

Growing population has resulted in a steep increase in demand for fresh water coupled with increased contamination from untreated waste water. Along with steps taken to clean our polluted rivers and streams, laws for disposal of waste water are becoming stricter, resulting in an urgent need for setting up facilities for treatment of sewage. There are several treatment options, each with its own set of advantages and disadvantages. Drawing from our experience in setting up and running sewage treatment plants across various locations involving multiple technologies, this paper discusses the major technologies for sewage treatment.

Key Words

Activated Sludge Aerobic and Anaerobic BOD (Biochemical Oxygen Demand) / COD (Chemical Oxygen Demand) Decomposition Dozing Pathogens Primary, Secondary & Tertiary Treatment Sewage Suspended Solids Water Pollution


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ARTICLE BODY

Introduction

Sewage, also known as domestic or municipal waste-water, comprises of grey-water (from sinks,

tubs, showers, dishwashers, and clothes washers), black-water (the water used to flush toilets,

combined with the human waste that it flushes away) along with soaps and detergents and

substances that get disposed of in the sewage system, like toilet paper. Sanitary sewers serving

industrial areas also carry industrial waste water. Depending on the design of the sewer system,

sewage may also include surface run-off water, increasing its volume.

Typical Components of Raw Sewage

Water ~99.5%

Solids ~0.5%

Organics 70%

Inorganics 30%

Proteins

65% Carbohydrates

25% Fats 10%

Grits Salts Metallic

Fig – 1: Typical Components of Raw Sewage

Sewage is characterized by its volume or rate of flow, its physical condition, and its chemical, toxic &

bacteriological content. Sewage is mainly bio-degradable and most of it is broken down in the

environment. However, the process is slow, and un-treated sewage may contaminate the

environment and cause diseases. Sewage may also contain chemical and pharmaceutical

substances. Untreated sewage is suspected to be one of the causes of increase in antibiotic

resistance in many germs (“super-bugs”).

In urban areas as per CPHEEO standards, a single person typically generates around 80 to 120 litres

of sewage a day, with a dry sludge component of 50 to 60 g. With increasing population and

increasing urbanization, sewage disposal is fast becoming a major sanitary problem. In India, cities

and towns generate 38,255 kl of waste water daily of which only ~30% (about 11,788 kl) is treated

and the rest is left untreated. Apart from having limited sewage treatment facilities, many of

treatment facilities that do exist are not functioning properly.


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Issues with Untreated Sewage

While untreated sewage contains over 99.5% water, its other contents, namely nutrients (nitrogen

and phosphorus), solids (organic and inorganic matter), pathogens (bacteria, viruses and protozoa),

helminthes (intestinal worms and worm-like parasites), oils and greases, heavy metals (including

mercury, cadmium, lead, chromium and copper) and toxic chemicals (including PCBs

(polychlorinated biphenyls), PAHs (polycyclic aromatic hydrocarbons), dioxins, furans, pesticides,

phenols and chlorinated organics used in bathroom cleaners etc. are responsible for contaminating

the environment and spread of disease.

Human exposure to the organisms in sewage-contaminated water takes place usually orally through

the mixing of untreated sewage with fresh water, or by skin exposure to contaminated water while

swimming, bathing etc. Oral exposure causes gastro-intestinal disorders (diseases such as diarrhoea,

e-coli infections, hepatitis-A, giardiasis, amoebic dysentery, cholera etc.), while skin problems could

occur from dermal exposure.

Also, discharges of untreated sewage leads to depletion of oxygen in water bodies which results in

death of aquatic organisms including fish.

The Need to Treat Sewage

The safe treatment of sewage is crucial to the health of any community and to maintain the life of

aquatic organisms. With increasing population, the improper disposal of domestic sewage and

human excreta has posed serious problems of health, environment and bio-degradation. Untreated

sewage often contaminates water bodies (lakes, rivers, and the sea). Sewage contamination

increases the concentration of nitrates, phosphates, and organic matter (found in human waste),

which serves as a food for algae and bacteria. This causes these organisms to overpopulate to the

point where they use up most of the dissolved oxygen that is naturally found in water, making it

difficult for the natural fauna of the aquatic environment to live. These bacteria can also lead to

spread of water-borne diseases in humans and animals that come in contact with the water. It has

been estimated that almost 70% of India’s fresh water sources have been contaminated by sewage,

leading to calls for urgent action (e.g. the Ganga Action Plan for Ganga-Yamuna rivers which are

among the most polluted in the world).

Environmental Risk of Using Untreated Sewage & Industrial Waste Water

Irrigation with sewage or sewage mixed with industrial effluents results in a saving of 25 to 50% of N

and P fertilizer and leads to 15-27 % higher crop productivity. Thus, a significant portion of sewage

bypasses any treatment and is used for irrigation or, worse, joins water bodies


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Fig – 2: Untreated Sewage flowing into the Mithi River in Mumbai

However, there are a number of limitations with regard to waste water treatment and reuse in

agriculture, such as the production of waste water when the crops do not require irrigation water,

the location of the plants compared to the land requiring irrigation, the match between the waste

water fertilizer content and the crop requirements, the risk of over-application, vigorous incidence

of weeds and insect pests due to, in general, low uses of pesticides in agro-forestry systems and

early dropping and softening of fruits, etc. Intensive land application leads to accumulation of salts

and heavy metals in the soil, odour problems, salt and colour leaching affecting groundwater and

downstream water quality, etc.

This leads to a decline in the land productivity along with a build-up of toxic pollutants, which

encourages the overgrowth of weeds, algae, and cyanobacteria and causes groundwater and

downstream water quality to deteriorate.(Table – 2).

Sl No Area Agriculture Use Risk

1

Dhapa Untreated sewage from Kolkata and waste from Tanneries

Sewage fisheries, Vegetables

Pollutants from sewage and tanneries, heavy metal pollution – Cr+6, Pb Zn, etc.

2 Musi River Untreated sewage & industrial waste water

Paddy, Jasmine, fodder crops, spinach, amaranths,

mint, coriander

Industrial pollutants, increased salinity, weeds

3

Keshevpur and Okhla Untreated sewage from NCR region, industrial waste

Vegetables: eggplant, okra, coriander in summer;

Spinach, mustard, cauliflower in winter

Industrial pollutants, increased salinity, weeds, choking of river by excess duckweed growth

Table - 2 Some Examples of Detrimental Effect of Using Untreated Sewage for Agriculture


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Benefits of Treating Waste Water

Effective sewage management is essential for nutrient recycling and for maintaining ecosystem

integrity. Effective sewage treatment results in separation of sludge and treated water which results

in:

1. Improving the environment through proper drainage and disposal of wastewater

2. Preventing floods through removal of rainwater

3. Preserving receiving water quality.

The sludge is a natural organic fertilizer and can be used to restore soil fertility. Treatment and

recirculation of treated water from sewage is needed to tackle the growing crisis of a lack of

adequate fresh water (Fig – 3).

Fig - 3: Projected Water Demand in India


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Water Guidelines – Legal Requirements

There has been a rapid increase in water pollution, caused by rapid urbanization and population

growth. The Central Pollution Control Board, CPCB, has published discharge limits for various

pollutants and the maximum BOD, COD, TSS of discharged water. A snapshot of the General

Discharge Limits provided in the CPCB website is shown in Table –1.

Sl No

Parameter Standards

Inland Surface Public Sewer Land for Irrigation Marine Coastal Areas

1 Colour & Odour Colour& Odoursmall as practical - * *

2 Suspended Solids 100 600 200 Waste water - 100

4 pH Value 5.5 to 9 5.5 to 9 5.5 to 9 5.5 to 9

6 Oil & Grease mg/l (max) 10 20 10 20

7 Total Residual Chlorine mg/l (max)

1.0 - - 1.0

8 Ammoniacal Nitrogen (as N) mg/l (max)

50 50 - 50

11 Bio-Chemical Oxygen Demand (5 days at 200C) mg/l (max)

30 350 100 100

12 Chemical Oxygen Demand mg/l (max)

250 - - 250

17 Hexavalent Chromium (as Cr+6) mg/l (max)

0.1 2.0 - 1.0

25 Dissolved Phosphorus (P) mg/l (max)

5 - - -

29 Bio-assay Test 90% survival of fish after 96 hours in 100% effluent

31 Iron (as Fe) mg/l 3 3 - 3

Table – 1 Extract from General Standards for Discharge of Environment Pollutants

In recent times, CPCB, MoEF(Ministry of Environment and Forests) and permitting authorities are

requiring more strict discharge limits. Many industrial facilities are now required to meet zero-liquid

discharge. This means no wastewater discharge is permitted and the industries have to recycle and

reuse the treated wastewater.

Notification issued by MoEF requires that municipal discharges must meet the stricter standards of

BOD < 10ppm, COD < 50ppm, TSS < 10ppm Total Nitrogen < 10ppm and Total Phosphorus < 2 ppm

and an effluent coliform limit of 230 MPN/100mL for urban areas. This implies new technologies

need to be installed to achieve new standards.

This paper discusses biological treatment technologies for STPs to achieve effluent BOD/TSS of

30/30 mg/L - which meets the limits prescribed under General Standards by CPCB as shown in

Table-1. Additional technologies need to be installed to meet the new limits.


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Treatment of Sewage – main technologies*

The most basic treatment of sewage is by collection in a soak-pit and allowing water to soak into

the soil. This is usually adopted for independent toilets. As polluting of ground water is a possibility,

this treatment is inadequate and currently unacceptable.

Treatment of sewage is based on a method provided by nature, i.e. by using microbial action. When

a steady consistent supply of air is pumped into a tank containing sewage which has been screened

to remove all floating debris and non-soluble contents, microbes which are present in it get

activated. These microbes are present in the sludge which makes up a substantial part of sewage,

and they consume the pollutants in the sewage while the air supply brings them to life and keeps

them alive and multiplying. Most aerobic processes for treatment of sewage are variations of the

above, “Activated Sludge Process (ASP)”. A sewage treatment plant (STP) based on this aerobic

process will consist of the following major stages of treatment:

• Primary Treatment (Screening): In this stage, raw sewage is screened to remove floating

debris/ insoluble impurities such as plastic bags, leaves, twigs, paper etc.

• Secondary Treatment (De-composition): In this stage, oxygen (air) is mixed into the sewage to

activate the microbes which decompose organics in the sewage and cause them to settle as

insoluble sludge (biomass). Water and sludge are separated, the sludge is removed and dried

for disposal and the water free from sludge is sent to a clarified water tank.

• Tertiary Treatment (Treatment of Clarified Water): Clarified water is filtered through a

pressure sand filter and an activated sand filter to remove any remaining suspended impurities

and a substantial portion of the BOD & COD present in it. Finally it is disinfected to kill all the

bacteria present in it by either chlorination or ozonisation or with ultraviolet light. This tertiary

treated water can be used to flush toilets, wash roads and yards, and for gardening.

Smaller STPs use variations of the more modern membrane bio reactor system (MBR). MBR is a

very compact waste treatment system that combines biological decomposition with membrane

separation of the sludge (biomass). The membrane compacts & concentrates the sludge, making for

a far more compact design than the ASP system described above: it combines the secondary and

tertiary treatment into one single step. Further, it produces far less sludge. It is also not so sensitive

to input load fluctuations unlike the ASP system.

The simplest treatment of sewage is the duckweed pond. In this extremely eco-friendly system,

sewage is allowed into a constructed water body where certain kinds of aquatic plants are planted

which absorb atmospheric oxygen and let this out through their roots thereby providing the oxygen

to feed the microbes which clean up the sewage. This treated water however can only be used for

gardening, due to the low pathogen removal of this system. Its major drawbacks are that it requires

a lot of land and can breed mosquitoes; the major advantage is that it requires no electricity to

operate if the flow of raw and treated sewage is by gravity.

There are many other specialized technologies, including process variations of the above, which

are not yet very popular.


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Our Experience in setting up/upgrading/running STPs

RUDIMENTARY TREATMENT METHODS

Constructed Duckweed Pond System

Fig-4: Constructed Duckweed Pond at a Semi-Rural Area for Sewage Treatment

We constructed a Duckweed Pond System to tackle the problem of untreated sewage from settlements in a semi-rural setup. The shallow pond, with a depth of ~1.5 m has a retention period of between 7 & 14 days. BOD and SS removal of up to 30 mg/l is achievable in such systems. The duckweed pond has a high mineral and nutrient removal rate due to the uptake of duckweeds.

Anaerobic Pond System

Sewage Water

from Septic Tank

(filtered)

Treated Water

Sludge


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Fig – 5: Duckweed Pond System

The principal advantages are very low cost of construction and low sensitivity to varying flow rates

and other seasonal fluctuations. Duckweed is a very hardy plant with simultaneous and significant

nutrient removal and yields vegetative matter with a high protein content (40%) which is a good

animal feed.

Fish farming in the pond is possible but has not been encouraged for other reasons. Low pathogen

removal due to reduced light penetration and high land requirement are the principal

disadvantages why the system is only suitable for rural and semi-urban settlements with easy land

availability.

We chose to make a duckweed pond to treat raw sewage as the location is outside our lease area

and hence the construction of anything with significant time, capital or maintenance requirements

was not possible. The overflow of the duckweed pond is further treated in an Effluent Treatment

Plant.


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Soak Pits

For unconnected toilets in areas with low population, an older technology of soak pits is popular.

Initially, individual septic tanks with soak pits were thought to be the right choice. The principal

advantage of such a system is ease of construction (especially for widespread colonies with low

population). The principal disadvantages are low rate of pathogen removal, risk of polluting ground

water and inefficient soaking due to unfavourable soil conditions, i.e. impervious rock layer, leading

to the overflowing of sewage. The soak pits also need to be cleaned (de-sludged) at regular

intervals. Nowadays, soak pits are not allowed due to the risk of polluting ground water, and over

time most sewage lines have been/are being connected to the Sewage Treatment Plant.

Fig-6: Waste Water Overflowing from a Soak Pit possibly due to the impervious lateritic soil.

TECHNOLOGY FOR SMALL STAND ALONE STPs

Packaged STPs

Small-sized Sewage Treatment Plants made of a combination of FRP & Steel are popular options for

stand-alone STPs. Such STPs are usually in the form of a compact FRP tank which can be used in a

decentralized manner for aerobic treatment of sewage water and is ideal for residential and

commercial complexes, hospitals etc.

The STPs incorporate membrane technologies with low operation and maintenance costs and

reportedly give a high (90%) reduction in BOD consistently. However, the sludge needs to be

handled periodically along with replacement of the (costly) membrane. The initial capital cost is also

quite high.


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This technology is most suitable where connection to the sewage system is difficult or not possible,

and for wastes (like hospital waste) where there is an increased risk of contamination. We had

evaluated a project for setting up a packaged STP for a hospital; however the project is yet to

fructify.

Fig-7: Packaged STP in the form of a compact FRP tank for decentralized treatment of sewage


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STP TECHNOLOGIES ADOPTED FOR MID-SIZED STPs

STPs Based on Membrane Bio-Reactor Technology

Membrane Based Bio-Reactor Systems (MBR Technology) set up at various locations

In this process, sewage decomposition takes place in bio-reactors, i.e. MBR chambers. The bio-

reactor comprises of a tank fitted with an aeration grid. The bacterial activity needs dissolved

oxygen to synthesize the organic matter. This is supplied by passing air in the form of small bubbles

from the bottom of the tank, which increases the contact period and also facilitates mixing, thereby

increasing the efficiency of oxygen absorption.

The bacterial population grows on specially designed carrier media (MBR media) which form an

integral part of the reactor system. The media are made of small polypropylene elements with a

very large surface area for the bacterial population to grow.


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The technology allows for rapid treatment and is suitable for small to mid-sized plants with small

variations in input sewage load. Maintenance of the membranes and the bacterial growth are very

important and thus these plants need to be run with skilled/trained manpower.

Modular STPs based on MBR technology

Fig - 12: 10 KLD modular type STP with MBR technology

TECHNOLOGY ADOPTED FOR MEDIUM TO LARGE SIZE STPs

The Submerged Aerobic Fixed Film Reactor Process (SAFF)

The Submerged Aerobic Fixed Film Reactor Process (SAFF) has a small footprint and effectively

treats dilute domestic waste water. The low and stabilized sludge produced eliminates the need for

sludge digestion.

The treated water can be used for agriculture and gardening or used to recharge ground water

aquifers, while the dry sludge is a good fertilizer in the rural community surrounding our colony.

However, the disadvantage is the need for primary settling to avoid clogging (the process being very

sensitive to the quantity of sludge, it can only accept relatively diluted sewage). Another


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disadvantage could be the relatively costly, proprietary Aerobic Fixed Film which needs proper

maintenance and thus skilled manpower.

This technology was chosen for the said location, as it is suited for congested, sensitive locations:

there is no smell and the technology is compact and has lower capital cost than competing

technologies.

Fig - 13:150 kld STP at a mining colony based on SAFF Technology

Filter Press


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TECHNOLOGY ADOPTED FOR LARGE SCALE SEWAGE TREATMENT

The Sequencing Batch Reactor Technology

Fig 14: 200 kld STP based on Sequencing Batch Reactor - ASP

The Sequencing Batch Reactor is a variation of the Activated Sludge process which is a proven and tested sewage treatment process all over the world for the last seven or eight decades. The process requires uninterrupted power supply for constant aeration and sludge recirculation. Reactor sludge levels also need to be carefully monitored and excess sludge needs to be withdrawn from the system periodically. Hence, due to its high level of automation, the capital costs of such plants are on the higher side and skilled manpower is required for operation. However, the process is highly efficient and can remove >90% of the pathogens (bacteria, viruses, faecal coliforms etc.) and BOD. This activated sludge process is operated in batches through auto-control, with aeration and settling in one tank leading to a lower plant foot print. Suspended solid removal is also high.


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This process is ideal for larger sewage treatment plants in congested and sensitive locations as there is virtually no smell. This sewage treatment plant was chosen for a colony of 2000 people as the plant needed to be located inside the colony. Other Technologies

Trickling Filter One of the oldest methods of treating sewage, this technology is over 100 years old. Being rugged and simple, the process requires very little monitoring. However, the limitations of the technology, i.e. the need for consistent effluent/sewage input quality, low rate of pathogen removal (from 20% up to 70%), and the risks of blockage of port and bio-filter due to excess biomass growth and the risk of odour and filter fly, rule out the adoption of this process in today’s environment. If adopted at all, it is usually adopted nowadays only for pre-treatment and is followed by Activated Sludge Treatment using any of the technologies described earlier. High Rate Trickling Filters are sometimes adopted when the treated water can be prevented from contaminating drinking water sources and used internally for gardening etc. The sludge needs to be separately treated in a digester and used as manure.

Fig 15: Principle of the trickling filter for treating sewage waste water


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Up-flow Anaerobic Sludge Blanket (UASB) The Up-flow Anaerobic Sludge Blanket process (USAB) uses anaerobic reactions (in contrast to most of the processes described earlier, which depend on aerobic reactions) to decompose the sludge. UASB reactors are typically suited to dilute waste water streams (3% TSS with particle size >0.75 mm). Biogas with a high concentration of methane is produced as a by-product, and this may be captured and used as an energy source, whilst forming a blanket of granular sludge which is suspended in the tank. Waste water flows upwards through the blanket and is processed (degraded) by the anaerobic micro-organisms. The upward flow combined with the settling action of gravity suspends the blanket with the aid of flocculants. Since the process handles diluted sewage, sludge handling is minimized. Similarly, power supply interruptions have minimal effect on plant performance and the process can absorb hydraulic and organic shock loading. However, the USAB process cannot meet the desired effluent discharge standards (as the output effluent is anoxic and invariably exerts a substantial and instantaneous oxygen demand on receiving inland water bodies or when used for irrigation) unless proper post- treatment is adopted (usually through an Activated Sludge based treatment process). Hence this process is not used today except for pre-treatment.

Fig 16: Principle of the Up-flow Anaerobic Sludge Blanket process for treating sewage


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Phyto Remediation and related technologies

Natural Uptake of Nutrients

Fig 17: Principle of Phyto Remediation and

Phyto remediation is considered to be a possible method for the removal of pollutants present in waste water and is recognized as a better green remediation technology than many other technologies. Plants like water hyacinth which grow very rapidly in water bodies are extremely efficient in absorbing nutrients. The quest of such plants for nutrient absorption provides a way for phyto remediation of waste water along with the combination of herbicidal control, integrated biological control and watershed management controlling nutrient supply to control plant growth. Moreover, as a part of solving wastewater treatment problems in urban or industrial areas using this plant, a large number of useful by-products can be developed like animal and fish feed, power plant energy (briquette), ethanol, biogas, composting and fibre-board making. However, large scale adoption of this relatively cheap and efficient technology is hindered by the risk of the uncontrolled growth of water hyacinth choking up all nearby water bodies.

Soil Bio Technology


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Soil Bio Technology developed at IIT Bombay is another recent technology used to treat sewage. The VEC patented CAMUS-SBT system produces results of >95% COD reduction using a soil like media in the presence of terrestrial ecology with plants as bioindicators.

Electro-Flocculation

Electro-Coagulation is a system in which contaminants are flocculated by an electrical process

into a size which can then be filtered, pressed and disposed of. This flocculation process is

based on electrolysis, and so called electro flocculation.

When the installation is switched on, air is blown through the water in the reactor vessel. This

is to keep the water in motion and to prevent solids sinking to the bottom. Every thirty

seconds an extra boost of “cleaning-air” is blown through the water. In a typical system, the

reactor contains Iron (Fe+2) and Aluminium (Al+3) electrodes connected to the cathode,

although other types of electrodes can be used. The voltage varies between 9 and 11 volts.

The electrodes connected to the anode are slowly converted into Fe203 and Al2(0H)3. The

resulting roughly formed flocks provide a place to which any contaminant in the water can

attach. The dissolved contamination is connected to the flocks and is transformed into non-

dissolvable particles. These particles can easily be separated from the treated water by

filtration.

While the electro-flocculation process can rapidly treat large amounts of waste water, the

cost of treatment is prohibitive, mainly due to the cost of the consumable electrodes and

electricity. Thus there are no large commercial plants using this technology for treating

sewage waste anywhere as on date.

Conclusions

Increasing population and rapid urbanization is increasing the stress on our scarce water resources.

Compounding the problem multi-fold is the fact that most sources of fresh water – rivers, lakes and

ground water are rapidly getting polluted by untreated sewage and industrial waste.

Sewage treatment has thus become of paramount importance and laws on waste water

management are becoming increasingly stricter. However, sewage treatment is complicated and

has capital and operational costs. In addition, while the basic science of treatment in most

treatment philosophies is more or less the same, the requirements of space, capital and recurring

costs vary across technologies. Also sewage characteristics and uniformity of load vary as do the

acceptable level of treatment, rendering some technologies more suitable than others.


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Proper selection of the sewage treatment technology is thus very important. This article has

described most of the popular technologies adopted for sewage treatment and the possible reasons

for their selection.

Acknowledgements

I would like to place on record my gratitude to my colleagues without whose help, the construction

of sewage treatment plants described would not have been possible. I would also like to thank my

company for help and support during the construction as well as for the kind permission to put my

thoughts based on the experience in the form of this paper. I would also like to thank Mr

Chandrashekar Shankar for inputs on SBT and thank Mr Indra N Mitra for reviewing this article and

providing immensely valuable inputs.

All thoughts and opinions are my own and do not necessarily represent company policy.

References

1. ENVIS Centre on Hygiene, Sanitation, Sewage Treatment Systems and Technology

2. Dr Arunabha Majumder, STP Technologies & Their Cost Effectiveness, All India Institute of Hygiene & Public Health, Govt. of India

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India, Indian Council of Agricultural Research, New Delhi, India,

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cities, 1999, Control of Urban Pollution Series: CUPS/44/1999-2000.

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http://cpcbenvis.nic.in/newsletter/sewagepollution/contentsewagepoll-0205.htm


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8. Guidelines for the safe use of waste water, excreta use of waste water, excreta and grey water, www.indiasanitationportal.org.

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12. UNICEF India, Water, Environment and Sanitation, http://www.unicef.org/india

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15. Saber A. El-Shafai, Fatma A El-Gohary, Fayza A.Nasr. , N. .Peter van der Steen, Huub J. Gijzen, Nutrient recovery from domestic waste water using a UASB-duckweed pond system, March 2006, Bioresource Technology 98 798–80.

16. Ranganathan, S S, Waste Water, India Water Portal, www.indiawaterportal.org

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19. Information gathered from discussions with various technology providers and EPC contractors etc.

http://www.healthissuesindia.com/

http://www.unicef.org/india

http://swachhbharat.mygov.in/

http://www.indiawaterportal.org/

Date post:	22-Jan-2018
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