The Quick and Dirty on Sewage Treatment
November 18, 2013
WEST WARWICK, R.I. — You may not want to know what happens to your waste once the toilet is flushed. Not to worry, the mostly underground process is somewhat straightforward — and not entirely gross.
Thanks to gravity and pumps, it can take as little as 18 hours for sewage to be processed at a treatment facility and released into a local river or other body of water. The methods vary by facility. Rhode Island has 19 major sewage treatment plants, plus four major industrial treatment systems.
At the West Warwick Regional Wastewater Treatment Facility, an underground, 42-inch pipe delivers the nonstop flow of sewage from 30,000 residential and commercial customers. Each day, up to 8 million gallons of wastewater arrives from West Warwick, as well as parts of Warwick, West Greenwich, East Greenwich, Coventry and Cranston.
The overall volume of wastewater changes depending on the time of day and precipitation. West Warwick doesn’t collect stormwater, but rain, runoff and groundwater invariably seep into the system through manhole covers and leaky pipes.
Raw sewage first arrives at the “headworks” building, where barrel-sized tubs filter large debris, such as cloth rags, feminine products and condoms. A grit tank also removes sand and other inert material.The flow then heads to open-air, circular clarifying pools to collect debris that sinks or floats, mostly fecal matter.
Attached odor scrubbers vacuum and treat gases to keep unpleasant smells from offending neighbors. Slowly rotating arms at the surface and bottom of the 400,000-gallon tanks push the waste through underground pipes into the filter press room. This highly pungent stage of the process squeezes moisture from the waste, sending the residue — called sludge cake — to an open-air warehouse. The sludge cake is then either shipped to an incinerator or mixed with ash to create a commercial-grade compost, also known as biosolids. This fertilizer is given away and used as a soil amendment for roadside landscaping or used commercially to feed shrubs, flowerers and turfgrass. It wasn’t until 1989 that the Environmental Protection Agency (EPA) banned the dumping of sludge in the ocean. Most of the state’s sludge was buried in the Central Landfill until the late 1980s and early 1990s when incineration became widely used.
Before the Clean Water Act of 1970 the remaining water was typically treated with chlorine and discharged into a waterway. The federal law set new standards for the treatment of pathogens and other pollutants, increasing the workload for sewage treatment plants. Testing occurs daily for pathogens and levels of pollutant, including metals and cyanide.
In West Warwick, liquid lime is added as the wastewater moves from the clarifying pools to one of six aeration tanks. The 280,000-square-foot tanks use forced air and microorganisms to breakdown the smaller, suspended solids.
Wastewater must move constantly so bacteria and microorganisms can be removed or reused. After aeration, the wastewater heads to secondary clarifying tanks, where microorganisms settle out and are collected.
The wastewater next travels through a series of eight filtration tanks to remove nutrients, including ammonia and nitrogen. Finally, the wastewater flows through an ultraviolet light system where powerful UV rays zap disease-causing pathogens and viruses.
West Warwick is one of three sewage treatment plants in the state with a UV system. East Greenwich and Bucklin Point in East Providence also use such a system. All other Rhode Island treatment plants use chlorine to kill pathogens. A follow-up treatment with a chemical additive is needed to neutralize the chlorine.
The wastewater, called effluent, is finally discharged through a gate into the Pawtuxet River.
The West Warwick facility was built in 1942 and fully modernized by 1993. Like most treatment plants, it was built in a low-lying area to ease inflows. It sits on a peninsula in the Pawtuxet River, making it prone to flooding. In the March 2010 flood, it was completely engulfed in water, resulting in the temporary loss of its treatment capabilities, some for several months.
The facility will be adding a new treatment process to meet new reduced phosphorous requirements.
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