We are on fire in the country due to increased drive to cover more areas with sewerage network and treat the collected sewage with more stringent discharge standards. While the discharge standards itself requires a separate discussion whether they are practically feasible or not, we would like to discuss few of the design related issues we have seen in many of the tenders floated recently across the country for new STP projects.
1) Peak Factor Design and Preliminary/Primary Treatment
The CPHEEO manual dictates the design guidelines for various unit operations and processes for various sewage treatment projects in India and all tendering issuing authorities are obliged to follow the design guidelines proposed in the latest revised manuals.
There are many issues which require proper clarification and revision, one of them is Peak Factor based designs for Preliminary and Primary units like Screens, Grit Removal Systems and Primary clarifiers. The manual indicates a 2 to 2.25 peak factor design criteria based on average flow rate.
Most of the tenders indicates that these pre treatment units must be designed at this peak flow and thus any plant must have a working capacity of 2.25 times average flow rate for these units along with a stand by unit for screens and grit chamber. Usually two working and one stand by units are selected. That means, your two working units of screens and grit chambers must have 2-2.25 Qav. capacity.
Here is the catch , when one considers design of a Grit Chamber , We find that We need to design it at a peak flow capacity and the retention time based sizing of Grit Chamber should correspond to peak flow which is around 60seconds to 3.1 minutes based on type of Grit Chamber design selected. Thus, at average flow of the proposed STP, the retention time within Grit Chamber unit will be just doubled and further when the plant is not even receiving half of the average flow, the Grit Chamber retention time would be further increased.
This is the scenario with most of the plants that they even struggle to receive average flow forget about peak flows. Under such circumstances, how could one justify the performance of Grit Chamber for removal of Grit and FOG?? Because as the retention time increases in Grit Chamber, there are chances of re-suspension of settled Grit and thus carryover with the incoming flow, also FOG removal can be also seriously affected.
Unfortunately due to our poor data collection mechanisms , there exist no record of proving or ruling out this question whether Grit Chambers are performing as per design at much lower flows and higher retention time or not. One simple solution could be that We can propose a design in which the total no. of units provided i.e two working plus one stand by must have peak flow capacity.
We are not sure how such design practices are actually affecting the plant performance as no information is available. But one thing is sure, under new discharge standards and stringent criteria to meet treatment goals, if there is high turnover of Grit and FOG to the Aeration basins, it would be difficult to develop a biomass which can be settled well to BOD/TSS < 10 ppm standards. FOG causes a lot of filamentous growth within Aerobic reactor and can have serious consequences with settling. So there might be chances that though all is well with the biological part of design, still it might be missing the TSS < 10 ppm standard and a major cause to this might be the peak factor based design of Grit Chamber units.
2) Biological Unit Processes Design
While biological unit processes are “technology ” driven designs which have their own pros and cons in present practices, We will deal with it separately due to a murky war going on for it. The key issue again here is peak factor based designs.
Recently we have seen few tenders mentioning that the biological systems too must have 2-2.25 Qav handling capacity which is very difficult to design and practically impossible to maintain the biological process to meet the BOD/TSS/TN < 10 standards both at average flow and peak flows.
Again when one considers, peak flow based designs, the organic and Nitrogen loading which serves as the design basis, gets double almost and thus the reactor sizing also gets doubled. While operating the biological system at average and further lower flow rates, the required conditions of biological operations falls outside the operating window one needs for BOD/TSS/TN < 10 standard and maintaining healthy biological life becomes very difficult.
For example HRT increases to two times than required, F/M ratios reduces drastically for the same amount of MLSS required for peak flow when operated at average or lower flows . SRT/MCRT also gets changed to higher or lower side based on the actual loading of plant. Under such circumstances, optimal microbial population development within the reactor becomes next to impossible task and thus there would be development of nuisance organisms which have poor removal efficiency and settling properties leading to violation of the proposed new standards.
Recently saw one tender which requires to treat 70% of the plant average flow within 9 hours.
We are not sure which technology provider and EPC company would bid for such projects?
Your comments and suggestions are highly appreciated.