Raw sewage, nitrogen, phosphorous, algae bloom, and dead fish seldom make media headlines in Virginia. However, the disturbing frequency and intensity of sewage overflows and spills into rivers is indeed worthy of media attention; the state of Virginia has generally had a sewage spill each month over the past three years. The state’s centralized wastewater treatment plants are stretched to capacity while operating under the pressures of a rising urban population and the effects of climate change.
Many of these plants which were constructed soon after the end of World War II, combine sanitary sewage and storm water in one piping system, therefore prone to overflow during severe weather events. In times of heavy rain or fast melting snow, mixed raw sewage, pet waste, fertilizers and storm water all flow untreated into nearby rivers. The overflows cause excessive amounts of nutrients such as nitrogen and phosphorus to be released, resulting in algae blooms in the rivers and the Chesapeake Bay, simultaneously reducing dissolved oxygen levels in the water. They also result in the release of disease causing pathogens into the main sources of drinking water.
In 2017, the American Society of Civil Engineers rated the Virginia waste water treatment system as a D+ or poor, emphasizing that the state has an urgent need to increase the capacity and efficiency of current systems to address overflows. The state must now consider alternate approaches to centralized treatment given the constraints on the availability of land and finance for expansions and upgrades to existing plants. Retrofitting existing storm water retention basins with new treatment capabilities and developing localized treatment options involving the recycle and reuse of separated grey water from black water at source, are two solutions that can help unload centralized treatment plants, increase efficiency, and prevent raw sewage discharge into natural streams.
Designed, built, and operated by mechanical, civil and environmental engineers, Virginia’s wastewater treatment plants enable the removal of suspended solids, biodegradable organic nutrients, and pathogens from waste water through a six stage process. The first stage of treatment involves the removal of debris that could potentially damage plant equipment. Wastewater then flows through a treatment stage that includes the physical removal of 95 percent of solids capable of floating and settling. The next stage of treatment is a process which removes pollutants from wastewater through biochemical oxidation, achieved through bubbling air or through mechanically mixing and aerating the wastewater. Subsequent treatment removes residual solids and other pollutants such as metals. Wastewater disinfection and residual solids disposal are the final steps.
There have been significant innovations in Virginia in wastewater treatment. Major examples include the separation of sewage and storm water systems in several cities, low flow toilets to reduce wastewater flow, and the use of biological nutrient removal systems to remove nitrogen and phosphorous. However, the Virginia wastewater treatment system is still not sized to handle the surges of storm water generated by rainstorms and the increased wastewater flows from the rising population; the centralized plants are large energy consumers accounting for a third of municipality electricity costs, and a number of plants only perform secondary treatment still needing to address nutrient removal. According to the ASCE report, Virginia will require USD 6.8 billion for wastewater treatment improvements over the next 20 years. Financial and space constraints which have imposed limitations on upgrades and expansion are encouraging alternate solutions, which can effectively reduce the burden on centralized treatment.
To handle excess storm water, some wastewater plants in Virginia have built temporary storage ponds and the excessive flow is diverted to those storage ponds bypassing the wastewater treatment plant. After the storm, it takes days or weeks to drain these ponds slowly and to send their contents back for treatment prior to safe discharge. A proposed solution would be to retrofit these holding ponds with mechanical floating movable aerators so that the ponds can serve dual purposes. The aerators would facilitate oxygenation as well as create controlled conditions such as oxygen rich zones that promote nitrification and oxygen starved zones that promote denitrification. They would substantially remove nitrogen contaminants that discharge into natural bodies of water. Due to the mechanical simplicity and low maintenance requirement, the incremental cost of retrofitting these storage ponds to perform primary and secondary treatment will be far less than adding new capacity to existing space constrained centralized plants or building new treatment plants. However, the treatment in the upgraded storage ponds would still not remove certain challenging contaminants such as heavy metals which require advanced treatment such as chemical precipitation, adsorption and separation processes. Wastewater treated in the storage pond when subsequently channeled back into the centralized plant would result in a reduction of the treatment burden on the plant.
With the increase in population and urban sprawl, a second proposed solution to increase capacity, efficiency, and nutrient removal, would be to encourage a shift towards a more decentralized wastewater treatment system. Currently, approximately 25 percent of wastewater is treated in decentralized units nationwide. This approach focuses on the collection, treatment and disposal of wastewater from communities at or near the point of waste generation. It would also serve as a source for wastewater reuse, particularly for non-potable applications.
Domestic wastewater involves black water predominantly containing feces and urine, and grey water which is gently used water from bathroom sinks, showers, and washing machines. Grey water makes up roughly 50 percent of sewage and may contain traces of dirt, food, grease, and certain household cleaning products. However, it is low in pathogens, and can be treated through small onsite modular treatment units through a process including filtering, biological treatment and disinfection with ultraviolet or chemical methods. Treated grey water can then be used closer to the source of generation, for toilet flushing and landscape irrigation. It would also enrich the soil with phosphorus and nitrogen. Re-use of grey water has been beneficial for irrigation and industrial use in states such as California and Florida.
The costs of decentralized plants are borne by local communities and paid for through a surcharge on sewer flows, as is currently implemented in Loudoun county. Reducing the flows of wastewater will help drive down the costs of construction and operation of the treatment system. Prior to treating grey water in the decentralized community units, a simple engineered system in every household could separate grey and black water to facilitate water re-use. Filtered grey water would be re-used in toilets and organic residues would be sent into a sewer system or to an on-site or off-site local treatment unit.
The system could be designed as a simple plastic rectangular holding tank with two compartments separated by an overflow baffle. Wastewater would enter the first compartment designed with sufficient hydraulic residence time to allow suspended solids to settle out. A sludge pump would periodically purge settled solids from the first compartment for downstream treatment preferably at a decentralized local treatment unit. Overflow water from the first compartment, clear of sediment, would fill a second compartment equipped with a pump that supplies pressurized water for toilets on demand. The second compartment would have an overflow port to allow discharge of greywater flows that are in excess of the recycle demand imposed by the toilet flushing needs. This greywater reuse system could potentially reduce water usage in a household by 25 percent by ensuring that all toilets only receive recycled grey water. The smaller and more concentrated black water stream resulting from this system can be treated in smaller decentralized wastewater treatment plants. Black water undiluted by grey water is conducive to anaerobic digestion due to the higher bio-solids concentration. A byproduct of anaerobic digestion, methane gas, can then be used in the co-generation of electricity or in household furnaces.
Retrofitting ponds would be akin to a band-aid solution; stepping up decentralized wastewater treatment would be a longer-term solution which is economically and environmentally more attractive as communities spread out. Smaller systems require less capital investment, reduce the load on existing sewer lines, save energy by taking advantage of gravity flow to transport wastewater, optimize the use of land, and reduce the demands on fresh water through re-use. Reducing risks and successfully increasing the amount of wastewater treated in decentralized units will depend on several factors including development of regulations on the safe re-use of water, effective remote monitoring and control mechanisms, realistic pricing for users, and financing.
Developing a system on technical and efficiency merits is only part of the solution to improving wastewater treatment in Virginia. Educating Virginians to preventing potentially harmful substances from polluting water will be important to protect health and the environment. The reuse of wastewater in particular, requires community level acceptance for such approaches to take root. Decentralization of wastewater treatment is a good complement to current centralized treatment as it will sensitize communities to their unique needs resulting in a greater awareness and engagement in water conservation, recycle and reuse.
References:
American Society of Civil Engineers, 2017. 2015 Report Card for Virginia’s Infrastructure. Retrieved from https://www.infrastructurereportcard.org/state-item/Virginia/
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Sullivan, P. (2016, October 10). Alexandria spews 11 million gallons of raw sewage into the Potomac each year. Retrieved from https://www.washingtonpost.com/local/virginia-politics/when-it-rains-in-alexandria-sewage-flows-into-the-river/2016/10/10/16bed5d2-8bd7-11e6-bf8a- 3d26847eeed4_story.html?utm_term=.c0b395794b99
US. Environment Protection Agency. May 1998. How Wastewater Treatment Works: The Basics. Retrieved from https://www3.epa.gov/npdes/pubs/bastre.pdf
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Virginia department of environmental quality, Commonwealth of Virginia. October 2017. Status of Virginia’s water resources: a report on Virginia’s water resources management activities. Retrieved from http://deq.state.va.us/Portals/0/DEQ/Water/WaterSupplyPlanning/AnnualStatusofVirginiasWaterResources2017Report_signed.pdf?ver=2017-10-13-102924-013