How Operators Are Making Smart Investments for Aging Sewer Systems

The composition of modern sewage is vastly different than influent from a few decades ago. Pump stations, headworks facilities and other equipment within water resource recovery facilities (WRRFs) were not designed to handle it. One of the biggest culprits is the increased use of flushable consumer wipes and other non-dispersible fabrics that end up in the waste stream. These items eventually clog pumps, pipelines and sensitive treatment equipment. Combined with the vast deterioration and disrepair of the North American wastewater system, municipalities require cost-effective, reliable solutions to deal with tough debris and protect downstream equipment—without the luxury of a total system retrofit.

The aging wastewater infrastructure in the U.S. is sorely in need of maintenance and improvements. The Clean Watersheds Needs Survey report to the U.S. Congress from the Environmental Protection Agency details the $271 billion needed within five years to meet the water quality goals of the country’s Clean Water Act. Unfortunately, sewage composition continues to evolve and wreak havoc on wastewater systems. The annual production through 2020 of non-woven fabrics—the material used to make disposable and flushable wipes—is forecast to grow at 5.7 percent. This fact, combined with lower flows due to water conservation, means challenging times are ahead. Several cost-effective solutions are available to help combat these challenges. The combination of dual-shafted grinders within pump stations and the addition of fine screening systems at the headworks is one solution that could provide the needed relief to aging infrastructure without a complete overhaul of wastewater systems.

Dealing with Disposable Wipes

Disposable wipes and other non-dispersible products are a modern-day curse for older wastewater lift stations and pumps that move sewage through collection networks. Lift station pumps are designed to handle traditional wastewater solids, with some handling spherical solids up to 3 inches in diameter—about the size of a baseball. The problem created by non-dispersing wipes and rags is that they form into large rag balls in collection systems, or they form mats on the fluid surface of wet wells. The issue is amplified when these wipes combine with fats, oils and grease (FOG), as well as hair, in the sewage to become indestructible masses of solids. In many cases, dual-shafted grinders have kept the plague of flushed wipes under control. With their low-speed and high-torque design, grinders have been used for years in collection systems to shred tougher solids such as wood, rocks or other debris that can find their way into sewer networks and cause clogs. Many engineers and operators are using grinders to combat the introduction of wipes into wastewater and keep pump stations working as designed. In the fight against wipes, grinders can be a cost-effective alternative to upgrading older wastewater lift stations. Pump upgrades meant to deal with wipes can be a costly proposition with new electrical requirements, hydraulic considerations and potentially lower efficiency. Plus, some pumps may not be able to handle tougher solids such as rocks or larger rag balls that can be shredded by a high-torque grinder. While conventional grinders cut products into longer strips that can reweave with FOG and hair, some newer grinders have cutters that will cross-cut the wipes and stringy material in two directions, making the debris small enough to remain pumpable.

Helping the Headworks

It is not just the older collection systems that suffer from disposable wipes. Outdated technologies at the headworks can lead to maintenance headaches and inefficient plant operations. Bar- or rake-style screens that use only vertical bars to collect solids are often required to remove large debris, but they are not suited for effective removal of wipes. Regardless of how close the bars are spaced, some allow an unacceptable amount of wipes into the plants. The results can be devastating for primary treatment and sludge systems. An automated fine-screening solution is one upgrade to keep disposable wipes out of WRRFs. Fine screens are designed with two directional screening panels typically made from perforated stainless steel or high-strength plastics. The two directional fine screens using 6-millimeter-or-less hole sizes remove smaller pieces of trash, such as rags, wipes, cotton swabs, plastics and latex that can easily pass through traditional bar screens. In-to-out fine screen layouts such as band screens or drum screens offer effective capture of solids because of their non-carry-over design, which is 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-millimeter perforations to prevent small trash and hair from fouling the membranes’ pores. One major consideration with fine screens is that the smaller openings will achieve much higher capture rates than earlier generations of headworks screens. This means they will discharge more screenings with higher fecal content than typical bar screens—sometimes twice as much. This makes screening management key. Innovative screenings washer compactor solutions have been developed that wash, dewater and reduce excess debris for landfill disposal. Some of the most effective washer compactors have integrated equipment that shred and precondition the screenings for washing and compacting. The result is maximum fecal removal and a much drier and compacted end product, as well as a reduction in odor and overall loads of screening waste. Proper debris screening and removal at the headworks is crucial for improving operational efficiencies, lowering energy and maintenance costs, and protecting downstream equipment that has become increasingly more sophisticated and sensitive to damage from smaller and tougher debris. This trash quickly becomes problematic for operators downstream, as it collects on aerators, causes scum blanket accumulation in digesters, clogs pumps and floats in final clarifiers. Increased removal of inorganic solids can benefit treatment processes from activated sludge to oxidation ditches, as well as MBR facilities and lagoons. However, one challenge with fine screening is the removal of too many essential organics, such as human-generated (fecal) debris, from the waste stream. The biological process in the treatment facility is more effective for handling these solids than releasing them into a landfill. While fine-screening technology offers superior solids removal and protection of downstream equipment, the fine screen can be damaged from heavy debris often carried by storm flows in combined sewer overflow systems. This heavy trash can damage the screening panels or overload the removal systems. Another viable option in the headworks is the installation of a dual-shafted grinder along with the fine screen to precondition all solids.

Downstream Improvements

Faced with aging headworks infrastructure and an inability to deal with the increase in wipes, managers of the wastewater treatment plant in Oconto, Wisconsin, took steps to address the problems. The plant had been experiencing inefficient performance and ongoing maintenance from its automated bar screen. After only five years in operation, the screening system was worn out and had lost several lifting teeth. The extreme gaps in the filter screen allowed whole rags and piles of fecal matter to pass through unobstructed. Along with the damage caused downstream, mangers faced overpowering odors at the headworks facility caused by the debris the screen pulled from the channel. The plant operator rectified these problems by designing a new headworks screening system for the 3.2-million-gallons-per-day (about 500-cubic-meters-per-hour) facility, with plans for more effective screenings performance and odor control. A fine band screen and a washer compactor with grinder were installed at the facility to fully capture and clean solids, producing a dry, compacted cake without fecal matter. The odor problem was eliminated, and the screen captured tough solids. Prior to incorporating the new washer compactor, the Oconto operators were removing a 50-gallon (190-liter) drum of screenings twice a day. After the installation, operators emptied the drum twice a month. Operators also have benefited from an improvement in downstream processes thanks to the fine band screen removing many of the troublesome solids at the headworks, resulting in a reduction in clogs and plugging problems. In the absence of full-scale funding to fully rehabilitate aging infrastructure, improvements like the ones at the Oconto plant can deal with the new sewage challenges. Grinders can be an effective solution for handling disposable wipes in lift stations. Fine screening solutions coupled with washer compactors at a plant’s headworks can provide an effective means of dealing with debris while protecting the other operating systems.