Transforming Sewage into Clean Water

On a cold November day in 2016, Superintendent Jeffrey Backman and Assistant Superintendent Roxanna Chomas of the Allenstown Sewer Department took several members of the Allenstown Budget Committee on a tour of the facility. The intent was to give Budget Committee members a better idea of how the facility operates and what factors contribute to the cost of operations and special projects.

We met in the break room and listened to an overview of operations. From there, we walked outside to the Headworks, which is the building that receives the combined flow of Allenstown and Pembroke.

A huge pipe crosses the Suncook River just upstream from the old Suncook Mills, runs underground, and then emerges at the Headworks. Due to the strong odor inside, we were told to breathe through the mouth (which helped but was not really healthy). We took a short look inside the building where large items and grit were being screened from the incoming flow.

Next, we went to the aeration tanks where a variety of bacteria was consuming the organic waste and other pollutants coming from the Headworks. You could say that billions of busy bacteria were breaking down the biological brew into benign by-products. The bacteria, especially the aerobic type, are the key ingredients in this part of the process because they produce the enzymes which help to decompose the organic waste.

We then crossed over the tops of the aeration tanks and the blend tanks. While walking on vented steel panels, we could see the dark waters churning below. At one of the blend tanks (part of the dewatering process), the top panels were open and an extension ladder lead down the empty tank. We were offered the chance to climb down and inspect the tank from inside, but no one took the offer.

Earlier this year, they completed a project which installed huge Vaughn pumps and a new fluid mixing system to replace the air pumps that stirred the tanks. Sliding enclosures were also erected to weatherproof the pump equipment. This project resulted in a significant reduction of odors being released into the air, and a reduction in the number of complaints about odors reaching nearby homes. Amazingly, the project was paid for with funds collected from the septage haulers.

Before going to the clarifier tanks, we stopped to examine the septage hauler building. This is the busiest place on site with trucks constantly entering and leaving. Back in October 2013, the four-bay receiving station was officially opened, making the Allenstown Sewer Dept. the only facility in New Hampshire to be able to handle that many trucks at once. The septage companies like it since it allows their trucks to get to the next customer more quickly. The Sewer Dept. likes it because they are able to collect more fees per day from the septage haulers. We were told that, so far this year, the facility had processed over 19 million gallons of hauled waste and that 80% of the total sludge came from the hauled waste.

The drivers have been trained to operate a kiosk at each bay and to unload their own trucks into one of four 5,000-gallon steel tanks (called "cans"). The grit settled out and the remaining septage was drained into the blend tanks. A few times a week, a hauler was paid to remove a full can and take it to a landfill in Rochester, NH.

At the lower end of the site, we inspected the 45-foot-diameter clarifier tanks. We watched the skimmers as they slowly raked the surface like the hands of a clock. It was here that the remaining solids and bacteria settled out. To maintain a proper bacteria population in the aeration tanks, most of the bottom sludge (called return sludge) was sent back to the aeration tanks. A smaller amount of the sludge (called waste sludge) was sent to the blend tanks for dewatering. Clear water spilled over the weirs (a barrier) along the outer edge of the tank and then flowed to the disinfection tank.

Our last stop was the disinfection building at the low end of the facility. This building contained a 1,500-gallon tank of sodium hypochlorite (similar to the shock treatment that is used in a swimming pool). Just enough of this chemical is dispensed to kill any bacteria still in the water, after which it is released into the Merrimack River. Last summer, an intrepid team attached a buoy to the end of the outfall pipe to mark its location.

When we asked about inspections, we learned that the Department of Environmental Services (DES) in New Hampshire periodically tested the outgoing water. They also inspected all documentation to ensure that the facility is compliant with the DES permit.

Upon returning to the main building, we proceeded up narrow stairs to the screw presses and some more mouth breathing (or just toughing it). Waste water from the blend tanks was being slowly squeezed from the sludge. The filtrate (water) was then drained to the aeration tanks. The compressed sludge dropped into a roll-off container at ground level for offsite disposal.

This dewatering process removed 70% of the water from the sludge so that the initial weight of 8.34 pounds per gallon was reduced to 2.5 pounds per gallon. That resulted in quite a drop in the cost of disposal, especially when we were told that they processed 20 million gallons of sludge annually.

Quickly returning to the office area, we met the administrative assistant, who operated the elaborate computer system that managed the hauled waste process. On what looked like a 40-inch monitor, there were separate windows that displayed live camera views of each disposal bay and also the truck scales. When a driver entered an ID at the kiosk, the system accessed that company's account. A flow meter at each bay measured the gallons delivered. When unloading was completed, the computer operator generated an invoice that was sent by e-mail to the septage company. (No waiting for the Post Office to deliver a printed bill.) The driver also delivered a paper copy to the office per state requirements.

Once back in the break room, Roxanna Chomas used beakers of sample wastewater to demonstrate some of the major steps in the treatment process. After a few final questions, the Budget Committee members offered their thanks for the very informative tour. We agreed that we now had a much better understanding of the processes and costs involved in keeping all the pumps, motors, machines, and equipment running smoothly 24/7.