Timely wastewater screen checks help safeguard against El Nino environmental stresses. By Michael Bambridge (M.E., Chemical Engineering), Managing Director, CST Wastewater Solutions
Water and wastewater treatment plants (WWTPs) are going to have to work harder as Australia’s weather patterns turn drier with the arrival of the El Nino forecast by the Weather Bureau to be with us by the end of this year.
One of their first lines of defence against process disruption, blockages and environmental spills will be WWTP headworks comprising mechanical screens, screening compactors, grit removal systems and grit washing systems installed in municipal and industrial plants.
The challenges for WWTPs during El Nino may include widely variable flow patterns, including extended lower flow rates bringing with them greater concentrations of solids, debris, and grit carried in the reduced volumes of inlet water. Headworks will be asked to efficiently process throughputs without clogging and environmental spills – and continue to do it with less on-site maintenance because many local authorities today can afford fewer specialist operations and engineering staff on call or on-site.
Prevention is better than downtime
Prevention is better than expensive downtime. Damage or weakness is not always obvious. The first indication owners have that their wastewater plant screens need maintenance or replacement is when they fail.
If they fail, sewage, untreated food waste, or organic and inorganic waste can seep through plant and neighbouring surroundings, including sensitive land and waterways, with potential production halts, OH&S cleanup risks, pointed questions directed at those responsible for preventing spills and potential serious environmental consequences for public and private entities.
Because screening headworks get the dirtiest job of removing all the lumps, chunks, strands, and unwanted foreign objects that can foul the often expensive, settling, clarification, treatment, filtration, and energy processes that follow downstream.
Why check wastewater screens
The inlet screens can look pretty good to the untrained eye, so the temptation may always be there to check and forget, to save a few dollars if there is no obvious problem. This set-and-forget approach can become a costly lesson for several reasons, including:
- Screens do deteriorate over time. They may have a design life of many years in normal service – but routine service and maintenance are needed to keep them in peak operating condition. This can be particularly important in sandy areas (noting that municipal councils’ WWTPs serve more the 80% of Australia’s population living within 50km of the coast. Many industrial processing plants fall within the same heavily populated areas).
- Screen technology does evolve. What was a fine mesh screen 10 or 15 years ago, may no longer be up to the job of protecting newer processes downstream. Older, coarse screens used to remove large solids, rags, and debris from wastewater typically have openings of 6mm or larger.
- Beware particularly of grit. Wastewater contains large solids and grit that can interfere with treatment processes or cause undue mechanical wear and increased maintenance on wastewater treatment equipment. Preliminary treatment removes these constituents from the influent wastewater. Typical opening sizes for fine screens are 1.5 to 6mm. Very fine screens – such as those proven in Australia by CST Wastewater Solutions – have openings of 1.5 down to 0.2mm.
- Asset management. Inlet pre-treatment, including screens, is a robust but finite asset we all depend on in the waste management business. The value of this asset should be maintained by regular servicing, just like a car. CST has some screens still operating after more than 25 years because the companies involved have invested in cost-effective regular maintenance to extend product life and effectiveness.
- Risk Management. Because screens are your front-line defence, they prevent or limit damage in expensive downstream processes. These processes can be hard to get to if they do fail, then cause many more OH&S issues to rectify. Compounding the issue is the fact mentioned prior that many remote and smaller public and private waste treatment plants these days do not have the engineering manpower on hand to rectify unexpected or complex and expensive events quickly.
We know how important risk management is because we engineer and locally manufacture dozens of waste management (and waste-to-energy) processes purpose-built to minimise maintenance. The advantage of local manufacture is that the product is built to local standards (some of the best in the world, recognised internationally) for arduous local conditions. And when they do ultimately need maintenance, having achieved top lifecycle value, local manufacture means local service is available without stretched supply lines.
Most companies these days are risk-aware. They know production stoppages are costly, and environmental spills can affect their statutory operating licence. But we still need to see better maintained plants reasonably often causing issues for the operator. These should be a thing of the past when small investments in yearly maintenance and check-ups can return good value and ROI throughout the long term.