The Karnataka State Pollution Control Board (KSPCB) released a guideline for Sewage Treatment Plant (STP) design and location on March 1, 2021. This article, the second in a three-part series, analyses the provisions of this guideline.
In a nutshell, the guideline is riddled with technically wrong, poor recommendations.
If this guideline is forced on builders and STP manufacturers, they will end up producing substandard STPs that will be too expensive for users to run, and their frequent failures will result in even more expenses for builders and RWAs alike. This will also create huge friction between builders and RWAs about who will foot the bill.
The new guideline creates pressure on the KSPCB’s regional officers who, while knowing it to be completely wrong, have to enforce them on builders. And since the guide itself allows them to adjust the requirements, the implementation would be completely arbitrary and ad hoc. Such unfettered powers to arbitrarily change the yardstick encourages bribery. Therefore, such subjectivity must be removed.
KSPCB should have consulted all stakeholders — such as renowned STP designers, builders’ associations (CREDAI and BREDAI) and residents’ associations (BAF, BANA, etc) — before making such a sweeping set of requirements and recommendations.
1. The guideline recommends Activated Sludge Process (ASP) only above 500 KLD. which would make it unsuitable for most private STPs. However, ASP is the predominant STP type in this sector. It is the best technology in this range. It has a robust design, with the smallest footprint, is cheapest to run, reliability that is not sensitive to any external factor and is simple to maintain.
2. The guideline recommends Sequential Batch Reactors (SBR) and Moving Bed Biofilm Reactor (MBBR), but these types are not recommended.
- SBR is not recommended because it demands much larger footprints compared to ASP: It needs to have a 60% larger equalisation tank, a 50% larger SBR tank (compared to the aeration tank of ASP); and a 50% larger sludge-holding tank. Therefore it requires much higher energy for mixing and aeration.
- MBBR is not recommended because it is sensitive to solids in the sewage, and needs a primary sedimentation stage. Also, it needs Dissolved Oxygen (DO) of >4 (against 1.5 for ASP), which requires far larger air blowers, leading to much higher electricity bills. Its excess sludge is thin slurry, which is difficult to settle and to dewater. Finally, it is very difficult to maintain.
- Both SBR and MBBR have very thin sludge consistency, and so they need further treatment, such as thickening, digestion, and conditioning.
3. The secondary clarifier tank is the heart of STP like ASP and MBR. It has multiple critical design requirements. But the design guideline has not mentioned this tank at all!
4. The very first line in the guideline proposes the Equalisation Tank (Buffer tank) to be 8 hours uniformly for all technologies. This is fundamentally incorrect and technically not tenable since SBR is a Batch reaction process and the others are continuous treatment technologies. In the continuous treatment technologies, 4-6 hour retention time is found to be adequate to buffer the peak flows. And in larger plants, the larger number of sources result in smaller and broader peaks, which means they need an even smaller buffer tank.
So, if we design a large equalisation tank, it needs a larger footprint and also needs more air diffusers and larger air blowers. Thus, all the advantages of an SBR are washed out.
5. The guideline recommends a fine screen for all STP types. But in reality, only MBR STPs require a fine screen. For the other types, this will be an unnecessary headache, as this screen requires regular cleaning.
6. The MBBR STPs are extremely sensitive to solids in the incoming sewage, and therefore they require the primary settlement stage. This is not covered in the table.
7. Submersible pumps are not recommended as they have poor maintainability compared to centrifugal pumps.
8. Mixing during the denitrification stage by pumping back to the pre-aeration tank will not work at all: very little mixing happens in the main SBR tank, where the bulk of the bacteria resides. Unless there is good mixing between the substrate and the microbes, the denitrification reaction will not go to completion. This calls for a separate additional mixer(s) in the SBR tank, depending on size and configuration. This vital component is missing in the guideline.
9. For denitrification, there is no indication or guideline for Mixed Liquor Recirculation (MLR), which is the most critical design criterion for reducing nitrate. The MLR rate has to be rigorously determined based on (1) incoming Biological Oxygen Demand (BOD) level, (2) incoming Nitrogen level and (3) treated water Nitrogen level to be maintained. A detailed nitrogen balance around a carefully selected control volume will yield this figure. Thus the guideline fails to mention a crucial design method.
10. No guidance on how to size a sludge holding tank in the various technologies. This is a real concern since the criterion is different in different technologies. The amount and consistency of excess sludge differ for different STP configurations. Hence there can be no thumb rule for sizing the sludge holding tank. Also, it may be noted that the SBR and MBBR STPs have very low solids consistency, and therefore they need further treatment, such as thickening, digestion, and conditioning. The guideline does not mention the requirements for these extra components at all.
11. There is absolutely no logic in the proposal to store the treated water for 2 days, for use in gardening, because the STP produces practically the same amount of treated sewage every single day, including rainy days. Even if the demand for treated sewage drops on rainy days, the supply does not drop.
So there is no need to create a buffer quantity for this fluctuation in demand. Here, KSPCB will also have to take a clear stand on the draconian Zero Discharge Policy (ZLD), and clearly acknowledge that almost 50% of the treated sewage is excess, and it must be allowed to flow in the SWD network.
12. Polypropylene Plate and Frame filter press is the best and most optimal alternative for STPs less than 1000 KLD. The only reason for filter press not performing to the mark in a vast majority of installations is lack of knowledge and training in design, engineering, and operation and maintenance of the system, as explained below:
- Design: Must select the right size of filter plates and the number of plates, based on the estimated sludge quantity and the cake-holding capacity of the filter press. We have seen in a majority of cases that the FP is undersized and hence remains idle.
- Engineering: For relatively large size filter presses, one must provide a motorised, hydraulic pack-assisted closing device. If this is not provided, it is very difficult for the operator to use the filter press effectively. Therefore, the operator gives up.
- O&M: After every filtration cycle, the filter cloth must be scientifically washed to ensure the pores or openings in the knitted filter cloth do not get choked with sludge. For this reason, light brushing of the cloth, preferably with Sodium Hypochlorite (Hypo), is recommended to clear the organic fouling of the cloth. If this is not practised diligently, the filter cloth loses its ability to filter very quickly and the FP becomes dysfunctional.
Therefore, we believe, a sweeping diktat that filter press may not be used, is based on a poor understanding of the unit and an even poorer understanding of the above facts. The world over, filter presses have been widely used for dewatering sludge in mini and small STPs.
After the filter press, the next device of choice should bag dewatering units, especially for very small STPs. They are simple to use and inexpensive to run.
Instead of these choices, the guideline recommends solutions that are failures in real life:
- The vertical bowl type centrifuges fail to separate the sludge because the sludge does not have a high specific gravity
- The belt presses have limited capability only to thicken sludge and are not found in urban STPs
- The volute screw press is too expensive
- All of these systems also call for polymer addition, which, if not carefully controlled, can lead to disruption in the working of the biological system
13. Oil and Grease Trap (“OGT”): First of all, this is not a stage of STP, but attached to large kitchens. It is critically needed in malls (which have large food courts) and 5-star hotels (which have large kitchens). However, OGT has no utility in a typical residential apartment complex.
Typically, these OGTs are very specialised and custom-designed units. The guideline is too simplistic and does not convey any meaningful information about the OGTs.
14. Air blowers: designing for 2.5 kg of oxygen per kg of BOD will put the energy cost of an STP at stratospheric levels. It will also increase the size of the air blowers, cost of air blowers, noise levels and vibration levels.
This is a completely unwarranted requirement, and not based on any scientific criterion.
15. Ultrafiltration (UF) System: There is absolutely no logical, scientific or technical reason to mandate a UF system in an STP.
All present-day ASP STPs work reliably with simple and inexpensive Pressure Sand Filters (PSF) and Activated Carbon Filters (ACF) and they deliver a quality that is far superior to that specified by KSPCB.
Even if one accedes to KSPCB’s recommendation for the SBR and MBBR STPs, they will produce far more solids in the water, which can quickly jam even the more robust PSF and ACF filters. If UF filters are used, they will stand no chance.
A UF system is too expensive to run, and if it fails every few days, it is simply not worth it.
16. Programmable Logic Control (PLC) or Auto control of an MBR is not really an “advantage”: Its only function is to trip the plant if the membrane has a fault. It cannot extend the life of the membrane or prevent faults.
If the PLC System fails, an MBR plant cannot be operated in manual mode at all. And spares of PLC are not available easily. As a result, many STPs were forced to shut down for months when their PLC failed.
On the other hand, if the UF module is an add-on for the STP, it does not require a PLC/ Auto control.
17. The guideline recommends residual Chlorine of 2 ppm in the residual sewage. If this sewage is used for gardening, it will burn off the grass. The limit should be 1 ppm (ref: Environmental Protection Authority, USA.)
18. The guideline recommends ozonation, but currently, stable ozonation plants are not available. Most plants break down soon after installation.
19. Currently the ventilation of the STP plant is set at 12 changes of air per hour, which means the system can purge any buildup of gases within 5 minutes completely.
Against this, the guideline recommends 25 changes of air per hour. This is more than double, which means the ventilation system would become much more expensive to run.
Different standards around the world require 10 to 12 air changes per hour. Therefore our practice of 12 air changes is on the more conservative (safer) side of the range. Thus there is no need for 25 air changes per hour.
20. The guideline also makes it mandatory to fix real-time probes for each STP. That proposal has too many problems to be included here. Therefore, those problems are explained separately in Part 1 of this series.
21. The guideline recommends a modular design for STPs so that they can be run on a low quantity of sewage. Probably by “modular design”, the circular desires the STP to be designed in the form of complete parallel streams, so that when the sewage production is low, only a few of the streams can be operated at their full capacity.
However, please note that this situation is encountered in practically every large apartment (even if the complex is not built in several phases).
It is not always possible to design an STP in parallel streams. Instead, it would be best to sustain the low-population phase by allowing the complex to connect to the UGD, and charging them the normal rates.
This arrangement would also be useful if the STP has to be shut down for tank-cleaning (once in a year) or for major repairs (once every two years) or refurbishing (once every 5 years).
[Read Part 1 of the series.]
- “Fining apartment owners for faulty sewage treatment plants is unjust”
- What the journey of sewage tells us about lives and livelihoods in Bengaluru
- Averting Day Zero: How Bengaluru should manage its water
- Towards water security: How to set up a greywater treatment system
[Disclaimer: This article is a citizen contribution. The views expressed here are those of the individual writer(s) and do not reflect the position of Citizen Matters.]