What is the Purpose of Screening and Grit Removal for Sewage?
The journey of sewage treatment begins with preliminary steps to remove larger inorganic solids and debris that could damage downstream equipment and disrupt further processing. Screening systems and grit removal units achieve this crucial safeguarding against operational issues while enhancing overall plant reliability.
Effective screening and grit handling not only protects pumps, pipes and channels from clogging but also improves sludge quality. It reduces abrasive wear within settling tanks, digesters, and treatment basins. Considering the sheer volumes municipal treatment plants process daily, these first lines of solids separation prove absolutely vital. Let's learn how sewage screening and grit removal systems function.
The Role of Sewage Screening
Screening represents the initial solids separation step once sewage enters the headworks of treatment facilities. It targets the removal of coarse debris, rags, plastics and inorganic solids that could accumulate and obstruct flow paths. Efficient screening greatly reduces operations and maintenance requirements downstream.The most basic screening technology involves bar racks or trash racks – parallel steel bars spaced 1-3 inches apart, creating a vertical barrier. An automated rake system continuously removes accumulated solids off the bars for further processing or landfill disposal.
More advanced screening utilises finer mesh or perforated plates obstructing even smaller objects down to 6 mm in size. These improved designs capture hair, food particles, feminine hygiene products and other fine solids missed by coarse bar screens. Greater headloss does result, requiring periodic backwashing to maintain operational flows.For large high-flow plants, multi-stage screening progressively graduates from coarse to fine screening in sequential steps. Each stage further enhances capture efficiency while preserving downstream processes and equipment. Plants can integrate dual screening channels for redundancy and seamless maintenance, too. Overall, screening performance proves crucial for optimal sewage pretreatment.
Mechanics of Grit Removal Systems
While screens remove larger buoyant debris, grit removal focuses on separating out dense inorganic particles like sand, gravel, eggshells, bone chips and the like. If not separated out, these denser grits accumulate within pipes and channels, creating obstructive deposits. They accelerate abrasive wear, particularly on pump components too.
Grit chambers represent the conventional solution hydraulically facilitating the separation and collection of grit particles. There are many design variations utilised, but all essentially incorporate a regimen of velocity reductions and flows that direct heavier grits to settle out for removal. Mechanical equipment extracts settled grit for washing, dewatering and trucking off-site to landfills.
Some designs incorporate stacked tray shapes or tube modules configured in proprietary geometries, establishing optimal settling regimes. Facilities with space constraints may opt for compact square grit basins over elongated rectangular ones. Regardless of the exact configuration, the overall function remains reliably removing grit from raw sewage flows.
Grit Washing and Dewatering Handling
After mechanical grit removal, the collected slurry undergoes washing to remove organics and further treatment prior to disposal. Conventional grit washers or cyclones agitate the grit slurry, allowing lighter solids to separate out before final dewatering. Multi-tray travelling bridge filters provide another dewatering solution.
Adequately washing grit proves critical not just in removing foul odours but also for avoiding debris accumulation in downstream processes. At large plants, the volume of separated grit handled can reach significant tonnages annually, so efficient dewatering becomes essential to lower disposal hauling costs.Most plants aim to achieve a 50-60% total solids concentration minimum for removed grit before landfill disposal or other management options. Innovative approaches repurpose treated grit as supplementary cementitious materials for concrete production instead.
Screening and Grit Removal Performance
The effectiveness of sewage screening and grit handling systems directly impacts various sewage treatment plant performance factors. Operations staff diligently track percent solids removal, screening capture rates, grit quantities removed and overall headloss monitoring. Excessive solids bypassing pretreatment creates undue operational burdens and maintenance requirements.
It's crucial to strike the right capture balance without excessively impeding sewage flows upstream. For screening, most plants aim for 15-35% removal of coarse solids and 5-20% removal of finer particles below 6mm in size. Ageing screening infrastructure often sees declining performance with frequent jams and overflows requiring rehabilitation.
On the grit side, design standards target removing 95% of grit particles greater than 0.2mm in diameter. This preserves pumping equipment, tanks, and other downstream assets from accelerated wear or accumulation. Oversized or undersized grit chambers suffer performance shortcomings with higher solids carryover.
System monitoring and optimisations recalibrate screening and grit removal capture rates based on real-time incoming sewage characteristics. Control systems automate screening rakes, grit air pulsations, underflow pumping, organic return streams and more for stable performance.
Overall, successful screening and grit handling set the stage for all further sewage treatment by removing gross solids, inorganics and grit upfront. Plants enjoy reduced maintenance costs, capacity preservation and process longevity as a result.
Conclusion
While overshadowed by glitzier secondary and tertiary treatment processes, sewage screening and grit removal critically set the stage for all further treatment. Safeguarding equipment while preserving sludge quality from the start represents the ethos behind pretreatment objectives.
This blog surveyed how screening and grit handling systems function mechanically while noting their overall process performance impacts. We looked at emerging innovations streamlining and optimising solids separation, too. Continued advancements will only improve plant reliability and efficiency with this crucial first sewage treatment barrier.
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