What is wastewater and sewage?
Wastewater is the water that emerges after usage in domestic, commercial, industrial process and human activities. Sewage is the wastewater that is coming from domestic and municipal sources, without much of industrial activities. Human domestic water usage is in washing, laundry, cooking, bathing, flushing toilets, etc.
In the domestic water there are some clasification of wastewater,
- Blackwater- Separated toilet flush water
- Greywater/ graywater- water from bathrooms, sinks and laundry
- Brown water- combined toilet flush water and urine combined
- yellow water- separated urine mixed with some water
- indian kitchen sink water- this does not have a typical name as yet, and this has high content of food particles, starch, oil is drained into the sink, that otherwise should be disposed in a solid waste bins.
- sewage water- this is the combination of all the wastewater of above categories
There are technologies and options to treat all the above categories of wastewater separately, while a common sewage treatment is a widely applied option.
Why should a project avoid basement sewage treatment plants?
A basement generally is a area allocated for human activities such as car parking, and storage. It is also an area that is artificially ventilated.
A basement sewage treatment plant requires a lot of air for treatment and there is a lot of air that comes out from the treatment plant, both of which have to be artificially regulated in a basement sewage treatment plant. Such investment will become a very costly affair in the operation of a sewage treatment plant, sometimes just the ventilation system operation electricity cost is more than the plant running cost itself.
It is advisable to place the treatment plant in a naturally ventilated location, can be under the walkway, road or even under the open park area.
Is reuse and recycle of treated wastewater important?
Reuse and recycle is there to ensure compliance to treatment standards. When partially treated wastewater is reused, residents would come to know about it from the smell and water color and for their own sake will be vigilante and ensure their water is properly treated for reuse.
Dwindling water resources have forced communities to reuse and recycle treated water, there could be up to 50% saving in fresh water extraction by wastewater reuse and recycle.
Do schools, offices and companies need special design in wastewater treatment?
Most of the schools, offices and companies work in a single shift, schools even if there are 2 shifts the schools is closed for 12 hours. This has a direct connection to the amount of water comes to the wastewater treatment plant as all the water come to the plant collection tank in just 12 hours and has to treated in a day or 24 hours period.Sizing of the collection tank should be according to this. There are many failing sewage treatment plants due to badly sized collection tanks. As a rule of thumb, for a school or a single shift operation the collection tank should be at least 1 day to 1.2 days flow volume.
What are the reasons for foam in an activated sludge process sewage treatment plant?
There are 2 common reasons for foaming,
- During the start up phase- white foam occur when there is more food and less micro organisms in the reactor. This would subside with time (4- 10 days) when the FM ratio reaches optimal levels in the reactor. This is a positive sign of activated sludge formation.
- Dark foam in older plants- This is due to long sludge aging. This can be rectified by having an active sludge recirculation to anoxic chamber or removing of excessive sludge from the reactor.
There could be other reasons like excessive usage of detergent in new constructions during flooring and interior works. Care should be taken that the sewage treatment plant is not exposed to any abnormal condition.
Filamentous bacteria are the cause of foaming when foaming occur often. Technical expert review and control mechanisms are recommended.
Why there are minimum load required for starting a sewage treatment plant?
There is food required for the bacteria to form and live in stable conditions. The air supplied in the aeration chamber should be adequate.
In a classical continuously operated plant, the incoming water should be both adequate in flow and pollutant content (food for bacteria). During the start up period there is lower volume of flow into the aeration chamber, in other words the plant cannot be fed continuously. Therefore there will be either low levels of bacteria due to lower availability of food.
In a SBR plant, this situation is overcome by sequencing the batches according to the incoming water volume. For example, during the start up period, the plant can run one batch in a day by reducing the aeration period in multiple on and off cycles. By this way, both the bacteria population and the water load is distributed by means of a program. When the occupancy and flow increases, the program can be adjusted to operate more number of cycles in a day.
Who should design sewage treatment plants?
Qualified environmental engineering companies are the ones who should be engaged in the designing of sewage treatment plants.
Each sewage treatment plant requires different design based on type of usage and building type. Office, schools, colleges, resorts, communities, hospitals, industries and independent homes, each need specific design for the plant to work without problem. Most designers are unaware of difference of peak factor, peak load for equalisation tank design. There is a need to understand sludge volume index, MLSS, activated sludge, sludge age with respect to the plant type.
There is something called oxygen transfer efficiency for the choice and design of aeration grid and choice of compressor, which is determined by the water working depth and flow.
Although, not rocket science- it is better to work with technology companies that are expert in sewage treatment plant design to be sure that you get a performing plant.
When is the right time in the project to plan and have the sewage treatment system?
The choice of sewage treatment technology determines the space requirement for tanks and the electromechanical parts for the respective technology. Thus the choice of STP technology should be determined and decided at the planning stage of the project itself.
Depending on the type of building and excavation for basement and other underground structures, it would be better to complete the construction of the tanks for the STP during the excavation for the building if it is close to the building or in basement. This would save a lot of money that will otherwise be 3 to 4 times expensive if construction of the underground tanks has to be done after the construction of building.
The final stage of installation and commissioning could be done just before the occupancy of the building or even after the first occupants have come to the building. A well planned STP takes just a few days for installation and commissioning.
On the contrary, the choice of technology for a STP starts during the project completion stage and sometimes the tanks are in place with some design that does not meet the requirement to install any sewage treatment process and thus resulting in many non performing STPs.
Builders and Architects should choose the technology for the STP during the project planning itself as it would save the project money and more importantly the reputation of the project with a performing sewage treatment plant.
How does large sewage treatment plant and small sewage treatment design differ?
Large Sewage treatment plants are the municipal plants which are connected to sewer networks. Wastewater flows many kilometres before reaching the treatment plant. There is reduction of load during the flow and also losses due to evaporation, infiltration, etc. Moreover, the flow into the plant on a hourly/ daily basis would remain the same due to long distance sewer lines.
On the other hand small sewage treatment plants are localized to communities and it takes only minutes for the wastewater to flow into the sewage treatment plant. Also there is no losses in water as most pipelines are plastic and there is no loss of load that is expected in a large sewage treatment plant. The collection and equalisation should be a key factor in designing these small sewage treatment plants for proper performance.
Also there are varying loads depending on the building type. Schools and offices are closed on weekends, for which intelligent control systems and designs are necessary in small sewage treatmen plants, which are not so important in larger muncipal plants.
Should sludge handling be part of small STPs?
Small sewage treatment plants cannot be designed to handle sludge at site. The plant should be designed for a sludge holding capacity to retain spent sludge for months and years depending on the plant size. The spent and digested sludge can be then removed using a professional service provider to remove the sludge and send it a common sewage or sludge treatment facility for safe disposal.
Should sludge be removed to ensure proper treatment in small sewage treatment plant?
The answer is simply No!. The sludge wasting in the reactor is done as a part of treatment cycle. This spent sludge is automatically stored in the collection tank from where it can be removed after months and years. When there is too much sludge in the collection tank, there is a need for sludge removal, which can happen after months or years depending on the sludge holding capacity design in the respective plant.
What is done to the sewage treatment plant during low occupancy period to achieve consistent treatment?
When the low occupancy is for a long period, the plant operation can be adjusted by making changes to the controller software to run the plant at lower flow only. Also, EcoSave mode automatically detects low occupancy with the inbuilt sensing system during the low occupancy period or vacation. This is suitable for resorts, schools, colleges, theme parks, hospitals, hostels and offices which have low occupancy in weekends or vice- versa.
Who should give the performance be guarantee in a Sewage treatment plant?
It is a common practice for a MEP consultant to designs the plant and asks a vendor to supply material and provide performance guarantee. Logically, the designer of the tanks and process should give guarantee to the plant and not the vendor who is just a pump and material supplier and integrated. EcoTec plants are designed by EcoTec and thus the performance guarantee is also provided by EcoTec. EcoTec experts support consultants, architects and builders with design of the plants and with full performance guarantee for normal load, underload, and holiday mode operations.
Can a sewage treatment plant perform and produce higher treatment results with old design and with the addition of tertiary treatment?
Sewage treatment plant functions with air supply in a given time in a certain volume and flow to remove certain quantity of pollutants. When there is a new regulation the newly applicable standards should be applied to the design and the respective volume, time of retention and the air supplied should be adjusted to achieve the new treatment standards. Tertiary treatment is only to polish the treated water to give improved aesthetic appearance, while secondary treatment is where the real treatment happens. There are lots of vendors, consultants and companies that are recommending ultra filtration to have better quality of water, which is nothing but a white elephant for the customer.
Why treat wastewater and sewage?
water is polluted beyond nature's capacity for a self cleaning process in a given time and accumulation of polluted water creates a problem for the present and pose a bigger threat to the future generation and the environment.
Moreover, our water resources are getting scarce and there is an immediate demand to conserve the reserves and resource. Population growth and urbanisation is happening at alarming rates and Governments and local bodies are unable to provide the services of sewers and treatment of wastewater that is otherwise provided in a planned city. Thus, the responsibility is passed on to the individuals, small communities, industries to treat their wastewater to meet the stipulated norms.
There are industries and commercial buildings that produce wastewater that have special characteristics and need special treatment. Thus the polluter cleans principle is applied to commercial buildings and industries.
How to calculate and design a community sewage treatment plant?
According to the Indian guidline, 150 lts/ person of water is consumed in a residence in a day. This entire water reaches the wastewater treatment plant. There could be other extraneous water also resulting from leaking taps, seepage in pipelines, rainwater intrusion, etc. Any design should take these factors in to consideration while designing a wastewater treatment plant. Also typical consideration of BOD is 400 mg/l and BOD is 800 mg/l for design.
After this there is the collection tank design should take care of the varying flow, buffer and sludge accumulation and make liberal provision in the collection tank, especially in small community STPs. It is always advisable to keep the collection tank volume as the same size as that of total daily inflow.
Aeration is calculated based on oxygen requirement. There is a need for 1200 mg/l oxygen to remove the pollutant. This is present as 20% composition in air, and is diffused in to the wastewater at an effeciency of 2- 7% per meter depth depending on the type of diffusion systems.
The retention time in a chamber also determines the cleaning of water. As a rule of thumb, if the aeration chamber and the sedimentation chamber combined is the volume equivalent to the daily inflow, it is an adequate design.
The main factors influencing treatment are the capacity of collection, aeration and sedimentation tank. In SBR based plant the aeration and sedimentation happens in a single tank.
The other important factors influencing are the choice of pumps, blowers and filtration systems. Automated systems are more reliable that manual plants that have to depend on human operators.
Please write to us your specific questions and we will be happy to answer.
How to calculate the wastewater generated in different setups?
Depending on the type of building, its users and behaviour of people as inhabitants, visitors the volume of water used by a person can be derived and is termed as Population Equivalent (PE), 1 PE= 150lts/ day. Based on this a general guidline is developed for designers to plan for a wastewater treatment plant.
|1 PE =|
|hostel/ guest house rooms||1 bed (guest)|
|Fine dine restaurants||1 seat
|café with no cooking||10 seats|
|club house (no cooking)||5 persons|
|sports ground||30 persons|
|industrial unit||2 workers|
The above are guidelines, while consulting an expert is always the best option. Please give us the information of your project and we will help with right treatment design calculation.
what are the common terminologies used in sewage treatment process?
KLD- Kilo litres per day or cubic meters per day
PE- Population or Persons Equivalent (150 lts/ day)
MLD- million liters per day
PE- Population equivalent or Person Equivalent
BOD- Biological/ biochemical oxygen demand in mg/day (the weight of oxygen required to remove the organic pollutant in one litre of water)
COD- Chemical oxygen demand in mg/day (the weight of oxygen required to remove the chemical pollutant in one litre of water)
HRT- Hydraulic retention time in hours (the time the incoming sewage is retained so that there is enough microbes in the system available with the supply of adequate air to remove pollutants as expected)
MLSS- Mixed liquid suspended solids in ml/l or mg/l (the amount of micro organisms in one litre that will be adequate to remove the pollutants in a given period of time)
FM ratio- The amount of food coming as BOD and COD to the amount of microbes available in the reactor
SVI- Suldge Volume Index- It is the measure of settle-ability of activated sludge in the reactor. It is an indirect measure of MLSS and performance of the activated sludge process.
Are Compact sewage treatment systems more suitable for small plants?
Any sewage treatment plant receives waste water that has oil, suspended particles and settable particles. Apart from this there are sometimes other undesirable articles like condoms, sanitary pads, plastics comes in to the plant. These materials are removed by screening process in large sewage treatment plants by employing mechanical processes and operators. When it comes to small sewage treatment plants these particles cannot be removed on a daily basis and have to be accommodated in the collection for a longer period of time before removal. Also , a small sewage treatment plant is exposed shock loads and low loads more frequently than larger plants. Thus smaller the plant, the proportion of holding capacity would be larger to overcome uncertain conditions and to operate the plant without the need for operator.
A compact small sewage treatment plant will always have more problems than liberally designed small sewage treatment plant.
If there are standards for designing STPs, then why are there so many failing to perform?
There are standards to designing the aeration chamber or reactor in a STP and most of the suppliers try to follow these standards unless they are STPs proprietary claims.
On the other hand there is no standards available to design collection tanks, equalisation tanks, treated water tanks. Apart from the tanks design each vendor chooses to give his own choice of pumps, aeration systems and equipments. It is also very common that consultants emphasize on design and vendors are used as material suppliers. Often so it is true that the fundamental parameters used in the design is also wrong.All these factors put together, there is many possibilities for wrong design, inadequate choice of equipment, etc, leading to many failures in plant operation.
EcoTec professionals ask you the right questions to give you the right design, resulting in very few non performing plants.
How to choose the right STP design and Technology?
It is rather easy to explain what one should not do in choosing the design and technology in sewage treatment.
According to the requirement and the experience consultants, architects and builders have with usage of different technologies of sewage treatment systems, they should be in a good position to choose the choice of technology for a customer.
Many sewage treatments have failed due to application of principles of large sewage treatment plants for smaller and mid size plants by plumbing (MEP) consultants. Plumbing consultants should not engage in the designing of sewage treatment systems if they are not qualified environmental engineers. Some consultants claim that they are neutral to technologies in sewage treatment. This means they do not know anything about sewage treatment systems. Builders and architects should use plumbing consultants to plumbing alone and not engage them in design and validation of sewage treatment plants for projects.
Finally plumbing consultants, once have chosen the technology option for the project, should ask technology providers to give their design for the plant and not use the method of fill up bill of quantities for sewage treatment systems that have not been arived with any basis for design.
There is a need for shift in the way STP are procured if right choice of plant design and engineering have to reach the customer.
What is KLD and PE in STP design and operation?
KLD- kilo litres per day or cubic meters per day. It is assumed that a person uses 150 lts of water per day.
PE- Population or Person Equivalent. Each person produced 60 gram BOD (biological oxygen demand) and 120 gram COD (chemical oxygen demand) in a day when engaged in defecation, urination, shower, clothes washing and other household activities.
When this is diluted or equalized in daily water consumption and disposal it become BOD= 400 mg/l and COD= 800 mg/l.
With these as preconditions sewage treatment plants for small communities are designed.
In the case of water saving appliances in buildings, the PE should be kept the same and the equalized load should be altered. For example, if a community disposes only 100 lts/ day, then the design basis would be for BOD= 600 mg/l and COD= 1200 mg/l.
It is important for designers to make these adjustments for a plant to perform.
How does depth of water in the tank influence the design of the sewage treatment plant?
Operating water depth is most important in choosing the right compressor- The oxygen transfer efficiency is dependent on the contact time the air bubble has with water. The deeper it is the longer the contact time and better diffusion of air and oxygen in water
With change in depth the surface area changes, this means for a smaller depth there is larger surface area and lesser oxygen transfer efficiency. So there will be need for larger compressor (higher flow rate) and also more number of diffuser to cover area and also to diffuse more air.
Volume in all case will remain the same; it is the depth and surface area that determines the compressor choice.
How often should the sludge be disposed from the EcoTec STP?
Sludge is collected and stored collection/ buffer tank for a long period. Sludge disposal can be done when required or as per the guideline given by EcoTec in the operation manual. Each plant is designed with its own sludge holding capacity.
How does the sewage treatment plant work when there is fluctuation in inflow during low occupancy period?
EcoTec plants are designed with sensors that constantly monitor the inflow into the plant. When no or under load is detected, the plant shifts to EcoSave mode where very little aeration is given to maintain the microbe population. When the inflow become adequate a normal treatment mode is activated. This insures the same treatment even at low occupancy period. Most importantly it saves a lot of money for the customer.
Should a sewage treatment plant be designed for average water flow/ pollutant load?
A sewage treatment plant must always be built according to the maximum wastewater inflow quantity and highest pollutant load expected. However, the plant can be adjusted to operate at varying flow volume and pollutant loads with some intelligent control mechanisms. Weekend, holiday and seasonal variations are no longer the issue in the present day plants with intelligent sensing systems. All the sewage treatment plants designed by EcoTec have inbuilt intelligence to alter operations for low load, vacations and weekends. Hotels and holiday resorts have a challenge in the incoming load due to different menu in the kitchen and different products used by guest in their bathrooms. Such plants have to be designed with a good buffering and equalisation. EcoTec has the experience in all such conditions and help customers to overcome these common issues.
How does EcoTec automated plant eliminate the recurring costs in operation and maintenance?
EcoTec automation and the choice of high quality components are the main reason for trouble free operation of sewage treatment plants. The control mechanism with error detection enable experts and service personnel to know the need for intervention even without visiting the plant. There is no need for regular operator, which saves a lot of money for our customers. The replacement parts are very few and regular maintenance and replacement of wear and tare parts with good service support ensures the plant performance at all times. Our customers know the difference when EcoTec small sewage treatment plants are retrofitted in renovated projects.
What are simple and effective ways to check performance of a sewage treatment plant?
There are many ways to check, but the most effective ways are human observation and intuition,
- There should not be too much smell from the aeration tank
- Treated water before chlorination should not smell
- Treated water when kept in a container for more than 2 days should not change colour
- Treated water when kept in a transparent bottle for more than 2 days should not form algae
Apart from this there are other ways to check the performance of the plant based on activated sludge characteristics,
- The aeration chamber water with the bacteria/ activated sludge should be in brown color (light- dark brown)
- Activated sludge should not be grey, black or yellow in colour
- Water collected from the collection tank in a 1 ltr bottle should be brown in colour and when kept for 30 minutes should form clear water on top leaving the brown particles (activated sludge) to form flocks and settles to the bottom of the bottle.
- There should be a grassy smell in the water collected from the aeration tank