Stories from DEP is a collection of feature articles
published in DEP's internal newsletter, Weekly Pipeline.
This article was originally published January 4, 2011.
Digesting the Wastewater Treatment Process
At DEP we are proud of our ability to provide nine million New Yorkers with one billion gallons of water a day and our ability to treat even more than that — an average of 1.3 billion gallons a day of wastewater from domestic and industrial use and storm runoff. Untreated wastewater can endanger public health, degrade the city’s surrounding waters, and cause unpleasant sights and smells. However, New York Harbor is the cleanest it has been since the Metropolitan Sewerage Commission, DEP’s predecessor, began testing harbor conditions more than 100 years ago, and with the progress of upgrades at Newtown Creek, all 14 of our in-city wastewater treatment plants now meet secondary treatment standards mandated by EPA. This has happened because of a multi-step treatment process that keeps our waterways clean.
The treatment process includes physical, chemical, and biological processes that remove at least 85% of pollutants and disease-causing pathogens from wastewater. The first step of the process is preliminary treatment, where a series of grates called bar screens remove solid objects — such as rags and pieces of wood that wash down storm sewers — from wastewater. Giant pumps then raise the wastewater to a series of settling tanks for primary treatment, another physical process in which the flow is reduced from a speed of two feet per second to roughly one foot per minute to allow heavy waste to settle to the bottom and lighter waste to rise to the top. Slow-moving bars skim the waste from the top and bottom.
Suspended material that neither sinks nor floats moves to another series of tanks for secondary treatment, or the activated sludge process. Much like bacteria breaks down food during digestion in a human body, in this process good bacteria consume the waste in an oxygen-rich environment. The bacteria become heavier, settle to the bottom of another battery of tanks, and are then removed. Finally, the remaining flow is disinfected with sodium hypochlorite — a stronger version of household bleach — before we release it into receiving waters.
Each step of wastewater treatment removes pollutants and impurities that we cannot release into surrounding waters; DEP has a parallel process to manage this waste, which is called sewage sludge. Primary sludge removed during primary treatment is de-gritted with a cyclone to remove stones and debris small enough to pass through the bar screens. The remaining sludge from primary and secondary treatment is thickened through the force of gravity so that the remaining processing is as efficient as possible; any water that thickening removes returns to the entrance of the plant and restarts the entire process.
The next step is digestion, where the thickened sludge is subject to bacteria in a warm, oxygen-depleted environment to render the organic matter more stable and less hazardous to the environment and public health. South Area Engineer for Wastewater Process Control Hayman Lochan says, “Anaerobic digestion operation is an integral part of wastewater treatment that breaks down as much as 65% of the organic content that our plants receive.” Another product of digestion is methane gas that DEP uses to generate power and heat for the treatment plants. Finally, the digested sludge is dewatered to further reduce volume; since not all plants can dewater sludge onsite, DEP maintains a fleet of three sludge boats to move sludge inexpensively from the plants without this capability to those with. Some digested sludge is also transported by pipeline. DEP recently issued a Request for Expression of Interest for new ways to use this material, including as fertilizer and a source of energy.