Water and Effluent Treatment Magazine
Issue link: https://read.utilityweek.co.uk/i/411335
20 WET NEWS NOVEMBER 2014 Ajay Nair reveals why he believes two-stage screening and planned maintenance is the answer. A solution: Screaming for screening P art of me thinks we should go back to screening plants of the eighties us- ing robust, two-stage screens. Not only will it allow us to operate plants effectively, increase their available capac- ity, improve effluent quality per- formance and reduce energy, chemical and maintenance costs, but there may even be a market for screenings. Never has the expression "one man's rubbish is another man's treas- ure" been so vividly true. We need to banish these Gremlins from our treatment plants without any hope of a sequel allowing them to make an unwelcome return. Get screening right is two- fold, firstly sizing of the equip- ment, including the ancillary items that wash water needs, control systems and the screens handling units, so they are suf- ficient to handle the peak capac- ities that occur at the start and end of storm events. The flow conditioning to and from the screens is vital because it avoids the potential to build up rag balls and presents flow at the right velocities. Planned maintenance I spoke about two-stage screen- ings, and this is something to consider though careful exami- nation of the network characteristics. Large, flat, gravity networks that are susceptible to gross sol- ids settlement during dry weather may have sufficient peaking factors to require a coarse first stage of screens. My view has always been that two-stage screening is worth its weight because of the importance of good screenings. Whilst initial, good design is vital without adequate mainte- nance, no matter how good the design, it is doomed to failure. Inlet works operate in an aggres- sive environment, no question about that, therefore it is vital that planned maintenance is carried out without compromise and that any reactive mainte- nance is kept at bay. Providing operators with suf- ficient standby capacity, the critical spares and easy access is also vital within design. It should be a no compromise situ- ation and the availability of screenings equipment should be the highest it possibly can be. In the future, as we move towards greater real-time under- standing of networks, we can perhaps begin to provide a link between screens availability and the risk of bypassing based upon predictive models linked to weather patterns for example. But before we do the new and exciting, let us get the basics right first and have a strong foundation to build off. I've seen the benefits of get- ting it right dozens of times. 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. A treatment plant serving the second city of Scot- land regularly attributes the improvement in profitability to the upgrade of the inlet works to a robust two-stage screening system. And I have two examples from colleagues in Asia Pacific where inlet works have been improved with the following direct impacts. At the Chapel Street WwTP, Tauranga, in New Zealand new screens have reduced opera- tional activities considerably in: • The pre-treatment tank (aerated grit chamber) was previously cleaned out every 12 months. It is now every 24 months • The aerated grit pump clogged regularly. It has not required cleanout / inspection for the past six months • Regular cleaning of screenings from the pri- mary settling tanks is no longer required • Reduction of rag accumu- lation in pumps in general and the anaerobic digest- ers has occurred • The screenings quantity has more than doubled A similar replacement was done at the Te Maunga WwTP, Tauranga, with major benefits to operation including no more blockages in grit pumps, raw sewage pumps, or at outlet dis- sipating wells in the final clari- fiers openings from energy. Give us a break To end I would like to say that although I am a technical direc- tor for MWH I have no vested interest in any sales of screening equipment. It is just that I have served my time removing screenings off the end of numer- ous DO probes and am pretty fed up with it and the missed opportunity of getting the pro- cess right. Although this is nothing compared to the brave operators who are forced to stick their heads into various pieces of equipment and pull out all man- ner of screenings with their hands! Come on people let's give them a break! Let's all scream for better screens. n Ajay Nair is a technical director at MWH. ajay.k.nair@mwhglobal.com INSIGHT Screenings are more akin to the post-midnight fed mischief makers, purposely destroying vital pieces of equipment used on treatment plants "Come on people let's give them a break! Let's all scream for better screens. "