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Correcting Steel Industry Activated Sludge Plant: Design and Engineering Related issues
So finally again a third article on the series. This improvement was not an easy task as we read here. It was a mammoth activity. Reviving biomass in a total volume of more than 80000 m3 reactor(s) is not a joke. It needs lot of patience. So there were lot of learning curves for us too.
Today we will discuss some of the interesting engineering related problems we found in this particular ETP. Now that remember, engineering is such a cost oriented aspect of a project and most of the time the nexus of Consultant, client and the EPC company dictate various aspects of engineering in such a manner that sometimes blaming one single entity for any specific blunder would be quite unprofessional. So read below suggestions with an unbiased mind and you will perhaps gain some insights in certain specific matters related to engineering of any wastewater treatment plant and thus could avoid major pitfalls during execution and operation.
Modularization of various unit operations and processes
One of the principle issues we had observed with this plant was single train configuration designed to treat the entire design flow rate. For a large flow of more than 440 m3/hr, the entire chemical pre-treatment comprising Flash Mixer and DAF system was designed with just one working unit of each flash mixer and DAF!
Now the main problem was that this ETP never produced that much high volumetric flow of effluent as per the design flow rate consideration. Thus, entire hydraulic system was a mess. Maintaining right flow rates across ETP became very difficult.
Usually flash mixers are designed with specific retention time based on the target pollutants required to be removed via chemical reactions with the specific coagulant and flocculants added in the flash mixers. So when the reaction times are changed due to lower flow rates ( two or three times lower than design flow) then maintaining right contact time for chemical reactions becomes very difficult and thus there might be chances of reverse reaction or release of higher amount of by products in liquid phase and thus not forming precipitates correctly.
While designing a WWT plant, due consideration must be given to minimum and maximum flows both. And thus modularization of the system is highly desirable step. It helps the process manager to control the process within right operating window or else maintaining specific process parameters would become difficult.
Similar issues were observed with the biological part too. Indeed it has two separate Pre Anoxic and Aerobic tanks. But they receive a combined effluent coming from the inlet and various internal flows like RAS and IR are also combined. So virtually it was a single system despite having two separate so called “trains”.
Another major issue with the project in biological part was a single secondary clarifier provided for the design flow rates and that was also designed at higher side of 6-8 hours HRT at average design flow rates. Thus, when the actual operating flow rate was nearly one third or two third of the design flow rate, clarifier retention time based on the actual average inlet flow rate exceeded to nearly 24 hours making the process management extremely difficult. Even at reasonable flow rates of 70% designed capacity running a clarifier at 10-12 hours HRT has serious ramification on biomass quality especially when one is dealing with toxic waste like integrated steel plant.
A well modularized plant having two or three parallel trains of plant having total capacity fulfilling the required design flow rate is highly desirable. When the actual wastewater generation varies then this modularization certainly helps in maintaining right process by at least taking one train offline. However, this could have significant cost consideration and thus many of the clients, EPC companies and even consultants prefer single trains of plants reducing the overall CAPEX significantly but ultimately when things go wrong, they pay dearly for the operation and maintenance of such process during the operation phases.
Think Operation and Maintenance during design and engineering: Maintaining right Process Parameters is key to successful design
It is our sincere advice to those who are working on civil, electro mechanical engineering and design side of the WWT that your sole focus must be on the process engineering. The civil, electro mechanical design must be incorporated in such manner that under varying conditions within the design limit of the plant, various process parameters can be managed as per the specific process requirement, Be it chemical or biological process, the outcome of the process is highly process operation specific and thus if one cannot maintain right operating parameters required for the process, We cannot achieve expected results. It becomes a costly adventure.
There are certain aspects related to ease of operation as well which must be carefully considered during the design and engineering phase of equipment and plant civil structure. The huge Aeration tanks of the bio ETP has three compartments, each are interconnected but there is no isolation facility for individual compartment and draining them for any maintenance work or for any other modifications. We have a large number of diffusers collapsed in the aeration tank due to scaling which was causing issues not only related to air supply but also due to poor mixing , keeping the biomass in suspension was a big problem. The system is designed in such a way that we cannot isolate any compartment individually and correct it for its diffuser issues!
Similarly, for the aeration piping also, a huge array of header and lateral piping has been provided with numerous drop pipes. But If we want to shut down air to any one compartment entirely and do some work then there is absolutely no provision at all! If one has to do some work in one main header and lateral pipe, then compulsory one has to stop aeration to entire aeration volume! It might be OK for few hours but if some work requires number of days to carry out then it is just not feasible with this plant.
Such designs are results of CAPEX “optimization” requested by clients and various associated stake holders. I would suggest that we as professionals should strictly prohibit such practices and need to make our stand clear that for optimum operation of the plant, certain minimum facilities and engineering arrangements are required regardless of its cost implications because on longer run, such cost cutting approach is recurring more cost rather than saving any money at all.
Virtually there are no flow meters in this plant except the inlet wastewater flow measurement equipment installed at the inlet pump outlet. Knowing various important flows like RAS, IR etc is just not possible. These are important process indicators when one is running a BNR system. But with good operating practices we could manage to run the plant and presently achieving fantastic results at a wide range of flow rates for the outlet quality. However, a good design and engineering incorporating various instruments like flow meters would certainly help for more sustainable management of the process.
Another design related issue was the bifurcating flows between the two Aeration tanks or trains from the central launder. All flows are received in a central launder and then it is divided between two trains using large valves provided. But again there are no flow measurement devices available to indicate which compartment is receiving what flow rates. It is solely at the mercy of the operator’s skills and observation. For a toxic effluent like Steel industry, a slight change in the distribution of critical pollutants like Cyanide between two compartments could affect the biological life in the receiving reactor quite adversely. So again, it seemed like “CAPEX Optimization” activity but with serious long term operational ramifications.
Anoxic Denitrification Mixers Arrangement and stand by provisions:
As BNR is new concept for Indian subcontinent and thus design/engineering issues related to Anoxic Mixing are yet to be fully understood and standardized, we would suggest the professional fraternity to consider the mixing arrangement and stand by provisions very seriously.
At this particular project, the number of mixers provided for various Pre and Post Anoxic tanks were inadequate in terms of their arrangement and stand by provisions. There were no stand by mixers provided and also arrangement was not optimum. The result was a serious deposition of sludge in the Pre anoxic Tank causing loss of active biomass. The worse thing is when submersible mixers are used and if any sludge deposition is happening, with the increase in the sludge deposition layer at the bottom (near the mixers), the load on the mixer would increase and at one point the mixers would stop functioning completely. So due to inadequate mixing arrangement, almost all active biomass was lost in the Anoxic tanks which required substantial effort to correct.
We leave it to the reader to absorb what it seems important to him/her but few simple suggestions we would like to summarize as below:
- Think always from process point of view and lay out your engineering design with a sole focus to manage right operating window for the ETP under varying operating conditions of inlet wastewater flows, chemical and biological parameters. Now days a gamut of simulation software available which one case use to find out operating windows under varying load conditions and this information can be effectively utilized during engineering.
- Modularize the civil, electro mechanical design so that the process can be managed under varying flow rates conditions. Providing at least two or more trains of entire process scheme is highly desirable.
- Don’t succumb to CAPEX optimization traits and stand for your design and explain your rational to clients and associated stake holder correctly and technically.
- Try to include as many indicators and measuring equipment as possible for key process parameters and don’t compromise on this aspect.
- Don’t forget to include stand by equipment for your critical electro mechanical equipment which otherwise can’t be replaced or repaired easily!
- Spend time on any existing ETP/STP and learn about day to day operations and management of the plant for its various electro mechanical equipment and civil structures. It will give you good insight into difficulties and issues faced by a process operator. We have noticed most of the design, engineering and project professionals in WWT field are not keen to visit a site and spend significant time onsite to know how about the daily operation of an ETP/STP. Spend time on a working plant and believe us you will learn a lot for something to agglomerate in your next project.