Small STPs: Applications and Advantages
Kannan Pasupathiraj, Managing Director, EcoTec Engineers and Consultants

Small Sewage Treatment plants (STP) are fully automated self-operating electronically controlled treatment systems that can treat domestic sewage (black and grey water) from individual houses that are not connected to sewers, and achieve treatment quality higher than the threshold limits prescribed by regulation. The article highlights the significance of STP and shares insights into its applications and advantages.

Rapid urbanisation and industrialisation is happening in India, such quick growth also comes with its effect on the environment, especially with water and sewage treatment. Metropolitan and many other smaller cities are faced with acute to chronic water shortages. On one hand, there is high water pollution of ground and surface water due to discharge of untreated or improperly treated sewage and on the other hand, industries and dwelling complexes empty raw sewage into water bodies hampering government’s initiatives in rehabilitating natural water bodies.

Sewers systems are effective and efficient where water is supplied from a single source. The groundwater dependency is high and will remain the same, but peri-urban settlements are faced with chronic ground water shortage due to over exploitation. Rainwater harvesting schemes reduce the stress on ground water depletion, but scanty and higher runoff limit the effectiveness of rainwater harvesting. There is a need for rethinking to find more innovative approaches to improve and increase the groundwater recharge.

Under Ground Sewerage System (UGSS) and sewerage schemes are already suffering in various proposed cities and towns due to lack of space, shortage of manpower, lack of primary data, etc. The situation is expected to worsen, when drainages will be burdened with silt, storm water surge, vector infestation, accidents, morbidity and mortality arising directly or indirectly from the ill effects of sewerage schemes. On top of the sewers, there is also a need for erection, operation and maintenance of centralised sewage treatment plants. There are only a hand full functioning sewer systems complemented by properly designed and operating sewage treatment plants.

User fee collection for water and wastewater supply and treatment is interlinked. Water charges include wastewater collection and treatment. It will be practically impossible for municipalities to charge this when most of the water used is from bore wells at household level. Even with 24x 7 supply of water, people would resort to using bore wells to supplement non-potable water needs. User fee collection for such schemes will be further challenged by lack of willingness from public and lack of political will to burden the users, leading to failure of such schemes even before starting.

Government has successfully incorporated rainwater harvesting at each household and a similar approach could be enacted to treatment of sewage and waste water treatment starting from household to communities. Individual homes, with the floor space of 2000 square feet, should have treatment facilities for reuse of treated water and also to recharge ground water aquifer.

Small STP - for single houses to communities, offices, factories, hotels and hospitals - will be something to look forward to. These plants will be fully automatic and work independently without any operator and they would be like any household appliance such as fridge and washing machine. The treated water quality will meet the prescribed regulations and the treated water can also be sent back to nature for ground water recharge, a better solution to compliment rainwater harvesting, recycled and reused. Such small sewage treatment plants will replace existing septic tanks that produce Ozone depleting methane gas and are hazardous to human health. These plants are taking us much closer to eradication of human scavenging as it is handled by professionals.

When each individual home is fitted with sewage treatment plants there is no waste water. The same become a resource and all the water can be utilised for gardening as the water has all the nutrients needed for plant growth. The treated water can be used for watering the plants and vegetation directly and also can be used to recharge groundwater. This means that groundwater table is recharged at the source of water and thus adding to the environmental sustainability. This is a better option than rain water harvesting, which is only seasonal. These plants are mandatory in Europe and will become mandatory in India too. These systems will compliment government initiatives and also where there are challenges faced to provide sewers systems. The Reliability of the System Quite reliable, the system works independently without any requirement for human intervention apart from periodical yearly maintenance. Small STPs could support smaller communities and independent houses and a sewer free environment.


How can as SMALL STP be used in existing septic tanks?
With minimal changes or even with no change, small STPs can be retrofitted into already existing tanks. This means no need for new tanks and civil work for homes.

What is the difference between a septic tanks and small STP? What about the volume of water treated in STP?
A septic-tank works in the absence of air and produces an effluent that is septic, devoid of oxygen and full of pathogens. As small STP uses air to generate natural microorganisms to consume and digest harmful elements in sewage, the resultant water is comparatively safer for reuse and has positive effect on environment.

The entire water that comes into the treatment plant is treated and sent out in batches. This means there is no water wasted in the treatment process without recovery.

Is there odour or gases produced?
The treatment is aerobic and large volume of air is pumped into the sewage water for treatment. By this way odor is minimized. Aerobic treatment does not produce harmful gases like methane and hydrogen sulphide.

Would there be a need for human/manual scavengers to enter the tanks for maintenance?
There is absolutely no need for manual scavenging. The tanks are free from any need for humans to enter the tanks for cleaning. Moreover, the tanks do not have odor and discomfort that a septic tank creates. Professionals/ engineers (like servicing washing machine or an AC) can be engaged for regular maintenance and thus it will not be a manual scavenging activity.

Will there be sludge produced?
Any biological treatment produces sludge and so does the small STP. The sludge generated is already digested and stabilised to a high degree that is safe for disposal into the environment. Onsite sludge stabilisation through sludge drying beds is an option. Government and private parties can be engaged in emptying and disposal services where sludge can be sold as agricultural fertiliser. With small STP, there is more regularisation of the desludging due and high degree of professionalism can be brought into the disorganised and illegal tanker services available. The stabilised sludge with its high nutrient value can be mixed with compost and sold to farmers.

How small STP ensure that the water is treated properly?
Each plant comes with an inbuilt artificial intelligence which ensures that water treatment is the same and to a high degree at all times. Alarms are set to warn the customers on the need for maintenance and also when something goes wrong. By this way the customer can always get the plant attended to. There are also options of remote monitoring systems where there is a need for regular monitoring. Bigger plants can choose to have remote monitoring systems where many plants can be managed from a central location through internet.

What about troubleshooting and malfunction?
The plant has a recording system, where all errors are recorded when it happens. The service people can read the error directly from the plant or through remote monitoring and attend to the plant needs without physical inspection of the plant.