By Fritz Egger, JWC Environmental
The treatment process for all sewage starts at the same place — the headworks. Inlet works facilities are responsible for a variety of pretreatment processes, including initial debris screening and removal. High-quality solids separation at the headworks is crucial to improving operational efficiencies, lowering energy and maintenance costs, and protecting downstream equipment — particularly since processes have become increasingly sophisticated and sensitive to disruption and damage from smaller and tougher particles. To adapt to these advances in downstream technology and provide increased productivity through improved solids separation and capture, wastewater inlet works are moving to finer and finer screening methods.
Choosing the proper fine screen can feel like a daunting challenge. Engineers must take into consideration the type of screen, degree of protection, system operation, and waste disposal methods to ensure maximum capture while maintaining optimum flow and level control. Understanding the different types of screens, as well as their benefits and limitations, will result in facilitating efficient and relatively maintenance-free operation.
Choosing A Fine Screen
Fine screens are typically defined as solids removal devices that have openings in minimally one dimension of 6 mm (1/4 inch) or less. They can be further categorized into straight-through or in-to-out (center-flow designs).
Products that employ the straight-through methodology that finely screen in two dimensions are typically constructed using a round-hole perforated plate design. This configuration offers superior capture performance when compared to bar or step screens, which screen in only one dimension.
In-to-out fine screen layouts offer the best capture of solids due to their non-carryover design, and are constructed as either perforated plate band screens or cylindrical drum screens. These designs are better equipped to protect downstream high-tech treatment equipment, such as membrane bioreactors (MBRs). MBR manufacturers frequently recommend a band screen or drum screen with 1 to 2 mm perforations to prevent small debris and hair from fouling the membrane’s pores. In this regard, flat plate membranes are a bit more forgiving than hollow-fiber types.
While fine screening technology offers superior solids removal and protection of downstream equipment, the screen is more susceptible to damage from heavy debris such as stones and irregular objects that are often carried by storm flows in combined sewer overflow (CSO) systems. This heavy trash can damage the screening panels or overload the removal systems. In CSO applications, multi-stage screening is typically employed, with a coarse screen placed upstream of the finer capture device. This approach employs a second solids removal area that will usually also require a washer-compactor to clean and dewater the removed solids.
A viable option that eliminates this preliminary coarse removal point and provides the additional protection for the downstream fine screen is to utilize a dual-shafted grinder to precondition all solids. This reliable and economical solution can shred especially tough debris to a size that can easily be handled by a fine screen and also by pumps in lift stations. The added benefit of a dual-shafted grinder is its ability to break down those essential soft organics so they are more easily passed through the fine screen, while controlling the particle size of the debris so the fine screen can still effectively remove the unwanted solids.
Finer screening at the headworks results in removal of greater quantities of solids than traditional bar screens — sometimes twice as much. The industry has developed innovative washer-compactor solutions that shred, clean, dry, and reduce screenings so they can be sent directly to landfill, often at reduced classification levels and cost. A key consideration of these washer-compactor solutions is selection of a system that not only compacts well, thus reducing volume and increasing dryness, but also one that effectively separates organics prior to compaction.
The following case studies demonstrate use of these state-of-the- art fine screen and washer-compactor technologies in action, and reflect on the cost savings, reduced maintenance, and downstream equipment protection associated with these capable technologies.
Revelstoke Treatment Lagoon — Revelstoke, British Columbia, Canada
The city of Revelstoke, British Columbia, experiences a population surge every winter as tourists flock to the area to enjoy the pristine scenery and excellent skiing and snowboarding venues. Since its opening in 2005, the Revelstoke Mountain Resort has hosted thousands of visitors throughout the winter months — which predictably causes a dramatic increase in the sewage that flows through the Revelstoke Water Resource Recovery Facility (WRRF).
In 1998, The WRRF installed a channel grinder/screen system in one of their three channels, thus processing (shredding) only 33 percent of the debris entering the facility prior to discharge to sewage lagoon. The problem was relatively minor prior to opening of the resort, but post-2005, the increased level of solids influx left operators struggling to skim floating debris off the lagoon during the busy season. They sought a solution that could keep the WRRF efficient and productive during these peak-flow periods.
The decision was to install a high-flow 6 mm perforated plate fine screen coupled to a washer-compactor equipped with a grinder to fully grind, clean, and compact the debris. Process improvement was immediate.
Oconto Wastewater Treatment Plant — Oconto, Wisconsin
Personnel at the Oconto Wastewater Treatment plant experienced inefficient performance and ongoing maintenance from an automated filter screen. After five years in operation, the screening system had lost several lifting teeth and was performing poorly in terms of capture efficiency. The increasing gaps in the filter screen allowed rags and other debris to pass through. Along with the damage this invariable caused downstream, odor levels had also become an issue due to the quality of the removed solids.
A new headworks screening system for the 3.2 MGD (500 cubic meters/hour) facility was designed, with plans for more effective screenings performance and odor control. Analysis resulted in selection of a center flow band screen and washer-compactor with grinder in tandem to generate high capture efficiency and efficient processing of the removed screenings. The odor problem was eliminated, and the desired level of solids was properly captured by the screen.
Prior to installing of the grinding washer-compactor, operators were removing a 50-gallon (190-liter) drum of screenings twice a day. After the installation, operators now empty the drum twice a month. Downstream process efficiency has improved as a result of increased upstream capture.
The Future Of Screening
Municipalities around the globe are continually working to maintain and ideally enhance the efficiency of their wastewater treatment facilities. The ongoing efforts within the industry to transition facilities from simple wastewater treatment plants to reclamation facilities producing reusable water will continue to drive the need for better process control, and as a result, fine screening.
Operation of reclamation facilities include not only wastewater treatment plants, but also sites processing food waste, fats, oils, and grease, along with biosolids. Facilities must be able to pre-process these inputs prior to feeding to high-performance anaerobic digesters or other systems to achieve the desired output or resources. Removing inorganic trash and rags out of the stream with fine screens while maintaining the organics loading is critical to the process.
As process sensitivity continues to increase, screening technology will continue to evolve and improve. Members of the Water and Wastewater Equipment Manufacturers Association (WWEMA) are well positioned to drive innovation in this area.
Fritz Egger is Managing Director, Asia, Pacific & Middle East Operations, at JWC Environmental, and is a member of the WWEMA Board of Directors.
Image credit: "Feinrechen - fine screen," SuSanA Secretariat © 2011, used under an AttributionShareAlike 2.0 Generic license: https://creativecommons.org/licenses/bysa/2.0/