Water and Effluent Treatment Magazine
Issue link: https://read.utilityweek.co.uk/i/411335
NOVEMBER 2014 WET NEWS 19 Not having a well performing screening process can bring chaos to treatment plans. Ajay Nair explains why screenings are not 'cute and cuddly'. The challenge: Banishing gremlins in screening Escaped screenings impact visually and take up valuable operator time to clean S creenings are unpleasant, smelly, stomach-churning and for many of us office- based design engineers, they are the most obvious reminder that we work in the field of sew- age. Which is probably why, the screening and grit removal pro- cesses are oen le to their own devices on many sites. However, not having a well performing screening process can have far reaching consequences with significant impacts on final ef- fluent quality, treatment plant capacity, operator morale and with growing importance today, on operating costs. You may well ask how screenings cause such chaos in a treatment plant? So indulge me for a moment and allow me to draw an analogy with one of the most iconic characters of the eighties, The Gremlins. Screen- ings rather than being the cute and cuddly Gizmo variety are more akin to the post-midnight fed mischief makers, purposely destroying vital pieces of equip- ment used on treatment plants just for the sheer fun of it (okay, I'm stretching the analogy, but you get what I'm saying). Let's consider where in the treatment process they do their most damage and the impact it can have on plant performance, cost and the morale (not to men- tion the health and safety) of site operators. Screenings that escape the inlet works first encounter the grit removal system – a poten- tial ally in the shape of medium to fine grit. By wrapping them- selves around scrapers and choking the de-gritting pumps, they can effectively prevent this process from working, and allowing grit to join the party of downstream destruction. Next up, the primary treat- ment process. For those of us involved in wastewater treat- ment design, the importance of well-designed and operated pri- mary tanks is a must. It reduces the organic load going forward and provides the best sludge for the digestion process to produce biogas. For every kilogram of COD that passes forward to the secondary process costs us 1.35kWh. Effective primary tank sludge removal is therefore vital and this is where mischievous screenings can really have an impact. Clogging of sludge removal pumps reduces the effectiveness of solids removal and leads to septic conditions in primary tanks (i.e. generation of soluble COD and ammonia), poor removal efficiencies and in extreme cases odour issues. Every one of these issues results in increased power and chemi- cal costs and reduced stability of the downstream process. The most valuable sludge is primary sludge. Its potential to generate biogas is much higher than that of biological sludge; losing it, means poorer quality sludge, less biogas, and could mean unstable digester performance. Clogging, choking, blocking Moving on to the secondary treatment process. Not content with increasing the load to this process, here screens can clog trickling filter arms, preventing rotation, or accumulate directly on the filter surface. Energy con- sumption increases because surface aerators become chocked reducing oxygen trans- fer efficiency. Screenings also wrap themselves around instruments preventing effective operation of the con- trol system –vital to both com- pliance and energy efficiency. The increased cost of remov- ing screenings from instru- ments adds to the already increasing operating costs and time available to the now stretched operators. Overall, a choked treatment plant can add 30-40% onto the power bill plus thousands of pounds onto the operator requirements. Aer toying with the second- ary process, any fugitive screen- ings can choke a tertiary filtra- tion process, causing it to bypass or go into excessive backwashing. Again, increasing energy and operator reactive maintenance and endangers compliance. But it does not end in the main wastewater treatment stages. Oh no. The impact of "The most valuable sludge is primary sludge. Its potential to generate biogas is much higher than that of biological sludge; losing it, means poorer quality sludge, less biogas, and could mean unstable digester performance." THE CONCEPT • Two-stage screenings so treatment plants can operate effectively and increase their capacity • Flow conditioning to and from the screens is vital to avoid the potential to build up rag balls • Greater real-time understanding of networks, could provide a link between screens availability and the risk of bypassing • The critical spares and easy access is also vital within design NEED TO KNOW 1 A recent survey of a treatment plant, showed that just by fixing the screenings issue, we could reduce the energy and chemical bill by £250,000 per year 2 A treatment plant in Scotland attributes the improvement in profitability to the upgrade of the inlet works to a robust two-stage screening system 3 A choked treatment plant can add 30-40% onto the power bill plus thousands of pounds onto the operator requirements. THE VERDICT "The time that the operators no longer need to spend defending against screenings can be put to use delivering further plant improvements, increasing capacity headroom and achieving a much lower cost to serve" Ajay Nair • INSIGHT FILTRATION & SCREENINGS screenings on sludge handling is probably even more problem- atic. Eventually screenings and grit end up in the sludge stream where they slowly grind away at pumps, reducing their effi- ciency, destroying seals and clogging up pipelines. Possibly the worst effect is the slow accumulation of grit in the anaerobic reactor vessels, which reduces effective perfor- mance of digestion, reducing biogas production and process stability. Poor digestion also means there will be poor downstream dewatering performance, increasing polymer consump- tion and the amount of sludge to be transported off site. Grit build-up can reduce the effec- tive reactor volume by more than 80% in extreme cases, and reduce biogas production to practically nothing. The cost of taking digesters out of service, cleaning and re-commissioning them runs into the tens of thousands. Then there is the loss of heat – creating the need to burn nat- ural gas, the loss of power pro- duction and the general uneasi- ness when it comes to opex review times. Messing with thickening The breakdown of thickening equipment is not uncommon due to screens. When it hap- pens, sludge storage tanks fill up, backing up sludge in pri- mary tanks spilling more load to the secondary treatment pro- cess. This in turn produces more unwanted secondary sludge. With nowhere to go it increases mixed liquor concen- trations, filling final tanks and causing blankets to rise. This leads to over-spilling of solids either to downstream processes, thus bypassing or increasing washing or going straight to the receiving waters. The ever increasing Mixed Liquor Sus- pended Solids MLSS drives up energy costs, at a time when lit- tle energy is being produced by the site CHP due to a lack of biogas. And the greater the biologi- cal sludge, the more unstable the digester performance. The only answer for frustrated oper- ators is to pay through the nose for a tanker to take the sludge away from site. The cost impact on sites can be huge, with unnecessary and reactionary opex. Meaning there is less money available to fix the actual issues that in some cases are as simple as replacing a penstock. A recent survey of a treatment plant, showed that just by fixing the screenings issue, we could reduce the energy and chemical bill by £250,000 per year. Ironically, the importance of CHP means there are dedicated teams in many water companies who are responsible for making sure their availability is high. I would argue the same approach needs to be applied to screens, because you may not have the gas to make those available CHP units work aer all. Inspite of our ongoing battle to try and lessen the mass of frankly bizarre items that end up at our treatment plants from customer homes, there will always be screenings to contend with. We know how to get it right and we should, in each and every case. n