The Wastewater/ STP FAQ, provides a primer on the basics related to all aspects of wastewater/ STP. The most popular FAQs are listed below. Please click on a topic to view more detailed information:
Waste water is the water that emerges after fresh water is used by human beings for domestic, commercial and industrial use. This document will restrict itself only to the waste water generated due to domestic use.
By and large,it is fresh water that is used for a variety of domestic uses such as washing, bathing & flushing toilets. Washing involves the washing of utensils used in cooking, washing vegetables and other food items, bathing, washing hands, washing clothes.
The water that emerges after these uses contains, vegetable matter, oils used in cooking, oil in hair, detergents, dirt from floors that have been washed , soap used in bathing along with oils/greases washed from the human body. This water is referred to as “ Grey Water” or sullage.
Water used to flush toilets to evacuate human excreta is called “ Black Water” or Sewage.
Grey water is easier to purify as compared to black water, i.e sewage. However, the practice predominantly followed in India is to combine these two wastes to discharge into a public sewer or into a sewage treatment plant in a residential community/ building that has no access to a public sewer.
As per standards laid down by the CPHEEO (Central Public Health Environmental & Engineering Organisation), the fresh water consumption per day per person should be between 135 to 150 litres per day. It is officially expressed as “litres per capita daily” (lpcd). By and large public water supply and sewerage bodies/authorities all across the country use the former figure to work out probable water consumption.
Waste generation in a residential complex:
When water is consumed by people living in a residential complex without access to an underground sewerage/drainage system , the amount consumed is estimated to be 135 lpcd. The total quantity (No. of residents X 135 litres) comes into a sewage treatment plant(STP) in the premises, and , this total volume has to be treated by the STP.
In a vast majority of cases, the actual waste generated exceeds this figure comfortably leading to overloading of the STP. This happens routinely because almost all residential complexes do not install water meters or similar water volume and flow measurement devices to keep track of water consumption in a residential complex/ gated community.
Consequently, when a device is installed and readings monitored, consumption has been found to be double and some times triple the suggested figure of 135 lpcd.
Waste generation in a commercial complex:
Human occupation of this kind of building is only during “duty hours”,i..e for approximately 8 to 10 hours per shift if there is more than a single shift. In this case water consumption is considered as 50 lpcd per person per shift.
Waste water contains all the dissolved minerals present in the fresh water that was used and which became waste water as well as all the other contaminants mentioned above. These are proteins, carbohydrates, oils & fats. These contaminants are degradable and use up oxygen in the degradation process.
Therefore, these are measured in terms of their demand for oxygen which can be established by certain tests in a laboratory. This is called Bio Degradable Oxygen demand(BOD). Some chemicals which also contaminate the water during the process of domestic use also degrade and use oxygen and the test done to establish this demand which is called Chemical Oxgen demand (COD).
Typically a domestic sewage would contain approximately 300 to 450 mg/litre of BOD and COD on an average. Sewage also contains coliform bacteria (e coli) which is harmful to human beings if water containing such bacteria is consumed(drunk). E coli is bacteria that thrives in the intestines of warm blooded creatures such as humans, animals and birds.
Another feature of sewage is the high level of Total Suspended Solids (TSS). This is what gives the sewage a black colour ,hence the name “ black water”. If sewage is allowed to turn septic, it then also has a strong, unpleasant odour.
Much of the water used for domestic purposes does not require potable ( suitable for drinking) water quality. For instance, water used for flushing toilets or for washing floors, yards or roads & gardening does not require to be potable. In a scenario where fresh water is getting increasingly scarce and when enormous volumes of sewage generated in the country are not being treated ,but goes unchecked to pollute fresh water from lakes, rivers and the ground water table, it must be treated.
Discharging untreated sewage into any drains other than an underground sewerage system, or into open land , is an offence and invites prosecution under the laws of all Pollution Control Boards in the country.
Sewage must necessarily be treated correctly and then re-used/re-cycled for various uses that do not need potable water quality. Recycling/re-using treated sewage can reduce fresh water requirements very substantially, by almost 50-60%.
In a scenario where fresh water availability itself is increasingly in doubt this is critical.
This requires plumbing to be laid so as to serve two sets of storage tanks on the roofs of any residential/commercial building. One set of storage tanks will be used to receive and store fresh water which will flow through plumbing laid to take it to bathrooms and kitchens where it can be used for drinking, cooking, washing & bathing.
The second set of tanks will receive treated sewage which will be connected by plumbing to all the flush tanks in toilets and to other points where the water can be used for washing yards, floors and also for gardening.
Sullage (grey water) which is mentioned above, if collected in a storage tank separately can be treated by aerating it to prevent it from turning septic, and then dosed with a coagulant, chlorinated and then subjected to filtration by pressure sand filtration followed by activated carbon filtration and stored in a separate overhead tank or tanks from which it can be used for flushing toilets and other uses where fresh or potable water is not required.
However, the current practice is to combine sullage and sewage (black water) and treat the mixture in an STP (Sewage treatment plant). This practice has come in predominantly to reduce the cost of construction of two separate plants and because space is now at a premium in any building.
From the point of view of a resident it is worth considering as it enhances the water security of the resident. A builder’s priority is totally different, since the space taken up by the treatment system can not be ‘sold’ to a buyer, he will just not consider it, instead the builder will combine greywater with sewage in an STP. This enables the builder to save costs.
However if looked at from the residents’ view point, a separate grey water treatment system being easier to operate provides a facility to ‘fall back on’ when the STP fails.
Why STPs fail becomes clearer as you read on.
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 in sewage, 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.
This is a time tested system called the world over as the 'Activated Sludge Process (ASP)'. It is the oldest system world wide and is the most used process world over. An STP based on this aerobic process will consist of the following major stages of treatment :
• Primary Treatment: In this stage, raw sewage is screened to remove floating debris/ insoluble impurities such as plastic bags, leaves, twigs, paper,etc.
• Secondary Treatment: In this stage, oxygen(air) is mixed into the sewage to activate the microbes which consume the pollution load and which then become sludge (biomass) . Aerated sewage and sludge are than separated so that the sludge can be removed and de-watered/ dried for disposal. The sludge can be used as compost in a garden. The water free from sludge is sent to a clear water tank(also called clarified water tank).
• Tertiary Treatment: 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 ozonation or with ultra violet light. This tertiary treated water is then pumped into a dedicated set of storage tanks from where it is used to flush toilets, wash roads, yards and for gardening.
A majority of the STPs in India are based on this ASP system in its most basic form. Such STPs are highly susceptible to input fluctuations (a frequent feature in India) and this results in a lot of untreated sewage and other related problems.
Other processes available :
The aerobic process as explained above is one where the microbes which clean up the sewage need to be supplied with air(oxygen) to function and multiply so that the sewage is ‘cleaned up’. An anaerobic process is one where a different kind of bacteria comes into action. It is a bacteria/microbe that does not need air and operates in an atmosphere without air ( hence the term ‘anaerobic’).This kind of bacteria produces methane and in the waste/environmental engineering industry, they are called “ methanogens”.
Mostly, anaerobic treatment is usually followed by the aerobic process and this combination is used where the waste water has very high values of BOD and COD. In such situations, the anaerobic system reduces the BOD & COD down to a level where the aerobic process completes the job of reducing it down to the levels where a tertiary treatment stage can do the final ‘polishing’ of the treated sewage as stated above.
The most common problems encountered are listed below and are based on an informal survey of STPs (including Water Treatment plants also) carried out over the last 4 years.
The tanks mentioned in 1) & 2) above are part of the primary and secondary treatment portions of an ASP system.
The prices given here are only indicative and meant to give an idea. All capacities given are in KLD ( Kilo litres /day, kilo= 1000 litres).
Also please note:
The operating costs, including maintenance for:
These costs do not include the cost of plant operating personnel. If O&M is provided by a reputed company which would use well trained operators with one operator per shift and one supervisor during general shift, they would charge approximately Rs.60,000. per month. Other obscure agencies who take on O&M contracts would charge approximately Rs.20,000.00 to 30,000.00 for providing the same number of personnel, but without the necessary training.
Yes, unfortunately, it is true.If sewage treatment was simple and easy for all to understand, this entire write up with FAQs would be much smaller and easy to follow.
Go to the “Ask the Expert” service and you will find that there are several persons with the necessary expertise based in different cities in India. Contact them and ask if they will help and the terms on which they will do so if it involves visiting your location. You will get one of them to definitely offer to help.
Yes, there are a few. Unfortunately, it is too late for Residents’ Associations to do anything about these because they take over a property after it is all over and done. It is the builders who should read this section and hopefully do what is suggested if they have their buyers’ interests at heart.
Something that is extremely important but never done till it becomes too late. A residents’ association must insist on the builder furnishing the association with all documentation of what the property has installed on it, eg, As built drawings with criteria used for designing/selecting, as well as detailed technical specifications for the electrical installations, power generation equipment, complete water infrastructure, piping for fresh water and sewage with drawings showing the routings and this must include the piping for the waste water to and from the STP.
Details of all the pumps and other motor driven rotary equipment as installed with information on how these have been selected, as, this will have a crucial bearing on the power consumption in the community. Proper documentation is a must for all the engineering incorporated in a property Here, ignorance is not bliss,it is an unmitigated disaster! Without this, maintenance/repairs can become a major problem due to sheer lack of information on all the equipment which is required to undertake planned maintenance.
One option is for the Government (either Central or State) to enact legislation to protect buyers of homes. Another option would be for the various associations of builders to themselves evolve a code of ethics that would ensure protection with regard to providing water security in a totally transparent manner. Neither of these is likely to happen in a hurry ,so, it may be necessary for Residents Associations countrywide to come together and put pressure on the governments and builders to ‘clean up their act’ and do things in a more transparent manner concerning the crucial aspect of water security.
Mr.S.S.Ranganathan, a Bangalore based water expert
India Water Portal requests users to view the same as a starting point in collating information on Waste water / Sewage Treatment Plants (STP) and to add more suggestions, information as responses in this thread.
Access here: Wastewater/ Sewage treatment Plant FAQs