Sewage Treatment Plant Interview Questions & Answers

Question 1. What Is Waste Water? How Is It Generated?

Answer :

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.

Question 2. How Much Waste Water Is Generated In A Residential Complex?

Answer :

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.

Question 3. What Are The Constituents Of Waste Water (sewage) ?

Answer :

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 Biodegradable 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 Oxygen 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.

Question 4. Why Treat Waste Water ?

Answer :

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/reusing treated sewage can reduce fresh water requirements vary substantially, by almost 50-60%.In a scenario where fresh water availability itself is increasingly in doubt this is critical.

Question 5. How Can Treated Sewage Be Re-used/recycled?

Answer :

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.

Question 6. How Is Waste Water Treated ?

Answer :

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.

Question 7. Why Not Consider Grey Water Treatment Seriously In Spite Of The Extra Space It Requires ?

Answer :

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 grey water 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.

Question 8. What Are The Problems That Can Be Expected With An Installed Stp?

Answer :

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.

Initial Start up of an STP failing to treat sewage: An STP is normally designed for the total sewage that can be expected when a building or premises is fully occupied. Full occupation in most cases usually takes up to a year or more. During this period when occupancy can be as low as 30% or so and gradually increases over a year or more, consequently the sewage that comes in initially does not provide the minimum load needed for satisfactory operation of an STP. It results in a situation which can only be called “sewage in sewage out”. Many STPs which face this situation take a long time to stabilize and provide treated sewage, very often, due to poor or wrong operation, STPs do not stabilize.

Poor design/under design of the STP : Often STPs which initially ‘struggle’ to overcome the first problem described above also can not function because

the balancing tank is undersized

• aeration tank is undersized or clarifier is badly designed
• the total inflow of sewage is higher than the volume the STP was designed to handle.

The tanks mentioned in 1) & 2) above are part of the primary and secondary treatment portions of an ASP system.Consistent mal-operation of the STP : Another very common feature is that a majority of plant operating personnel employed by agencies that take on Operation & Maintenance(O&M) contracts are illiterate, un trained and supervised by people with little or no knowledge of what O&M involves. Such agencies generally charge O&M charges that residents’ associations consider affordable. Companies with well trained operation personnel and experienced supervisory staff charge for services an amount that reflect their skill and expertise which residents’ Associations are reluctant to pay and thereby lose out on a well run/operated water infrastructure. They often do not realize that even the charges which they consider as cheaper/lower are going to waste if the sewage is only partially treated.

Strong smell/odour from the STP: This is a very common complaint from numerous housing communities and even commercial buildings which have an STP in operation. The smell is often very strong and quite often unbearable. It is caused by any one or all of the problems listed above. 

Very high noise levels from the STP: Quite often residents of an apartment building have sought help from experts to minimize the very high noise levels from their STP which they find unbearable throughout the day and more so at night, thereby preventing the residents from sleeping in peace.

Question 9. How Can Common Problems In Stp Be Avoided And/or Resolved?

Answer :

Modern designs for STPs which are modular are available from reputed companies which are in the field of water and waste water treatment. Such companies have standardized designs where,for instance an STP to handle 150 KLD ( 150,000 Litres per day) of sewage can be made up of 3 modular STPs each of 50KLD capacity. Such an installation would be able to handle the initial lower load of sewage with one module in operation with remaining modules being commissioned/started up as the sewage volume increases. Such a modular approach also makes it possible to handle sewage in the case of a break-down of the STP as it is extremely rare for all modules to break-down together. In short, there is a stand-by always available. For several years now a few companies have been offering microbial agents which can help overcome these problems if these microbial agents are added to the incoming sewage. Go in for Modular STPs & use microbial agents regularly.

 It is equally important to know and be able to control the volume of fresh water used in a community so that it does not exceed the design capacity of an STP. This involves installing water meters at all crucial points to measure water flow (consumption) & thereafter taking action to curb excess consumption of fresh water to prevent overloading the STP. Control excess consumption of fresh water and thereby prevent overloading of the STP

Builders are not expected to be experts in water or sewage treatment plant design, manufacture etc. They can however have tie-ups with reputed environmental engineering companies with sound technical experience and a proven track record, to make up for their lack of knowledge. This seldom happens since a builder’s interest ends with selling a completed project and then handing over the project to the Resident’s Association as soon as possible, often without even demonstrating actual, successful operation of the water infrastructure. Most builders link up with small, obscure local companies with inadequate knowledge and expertise in waste and water treatment,but will put up something for an extremely low price. The result is poor/ wrong operation of an STP leading to untreated sewage and unpleasant odours from it. Ensure supply of an STP from a reputed supplier and entrust operation & maintenance to a well trained professional team.

One of the major reasons for STPs not working properly is the fluctuations in input loads. Flow of sewage in a residential community is never uniform. It varies with peak flows in the morning (residents getting ready to go to work), very low or almost no flows later in the day with another peak in the evening. Raw sewage is collected in a sewage balancing tank(mentioned above) which should be sized to hold at least 6 to 8 hours flow of sewage. This ensures that the sewage collected in the balancing tank is homogenized, thereby avoiding input fluctuations in input load on the STP. Do not compromise on the size of a raw sewage balancing tank.

High noise levels from an STP are due to the operation of electric motor driven equipment such as pumps, air blowers, air compressors, etc. Old designs/makes of pumps, blowers , compressors , etc are still available at very low prices in the market and these are used in most of the STPs that have been put up. The noise levels of such equipment is very high as compared to modern, world class pumps and rotary motor driven equipment now available in India. These modern makes are almost noiseless and extremely efficient. The old designs are also the cause of high energy consumption in addition to very high noise levels. As per the laws in force in India, the noise level permitted in a residential area is 55 dB (dB= decibels of sound) during day time,i.e from 6:00 am to 10:00 pm and 45 dB during night time(10:00pm to 6:00 am).As compared to these limits, the actual noise levels are likely to be as high as 75 dB or higher. To reduce noise levels and high energy consumption, it will be necessary to replace most of the critical rotary motor driven equipment with the latest noiseless high efficiency equipment. Here it is advisable to choose a reputed company with an established reputation in sewage/waste water treatment to buy an STP. Such companies have constantly improved their designs to reduce the foot prints (space occupied) of their equipment and reduction in the power consumption of power by a very appreciable amount. Unfortunately, residents have no say in this as they face up to this crucial fact when it is too late as the STP has been ordered probably even before the residents bought a home in the property.

Question 10. What About The Operating Costs For An Stp?

Answer :

The operating costs, including maintenance for:

• STP of 75 KLD capacity and above :  1.2 paise per litre of sewage treated.
• STP of 50 KLD and below : 1.5 paise per litre of sewage treated.

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.

Question 11. What Can A Resident’s Association Do If It Needs Expert Help To Sort Out Problems With Sewage Treatment?

Answer :

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.

Question 12. Why Should An Stp Not Be In A Basement ?

Answer :

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.