Thursday, July 30, 2015

Our Sewers and Waste Water in the DC Region

The most essential infrastructure in our Washington DC Metropolitan region and anywhere are the portions that are almost invisible, but essential to the American way of life- water, sewage and power. The water infrastructure for the Washington Metropolitan region which provides our homes, schools, businesses and public buildings with healthy, reliable water and sanitation and plays a key role in protecting public health and maintaining and restoring the quality of our rivers and streams. There are three main types of water infrastructure: drinking water, wastewater and stormwater.

District of Columbia's sewage system is actually one of the oldest in the United States, and dates to around 1810, when the first sewers and culverts were constructed to drain stormwater and groundwater carrying sewage and muck from the streets of Washington D.C. Epidemics of smallpox, typhoid and malaria during and after the Civil War lead to an expansion of the system, though the system simply moved the waste from streets and open canals into rivers and estuaries. Washington DC did not start treating the sewage waste until 1937 when the Blue Plains sewage treatment plant was built. Blue Plains is still operating today when the treats to public health include MRSA (Methicillin-resistant S. aureus).

MRSA causes difficult-to-treat and potentially fatal antibiotic resistant bacterial infections that in the late 1990s began to appear outside the hospital setting in the greater community. The incidence of these so called “community-acquired” MRSA infections has been increasing in the United States. Scientist from the University of Maryland found that MRSA was present in 83% of the raw sewage samples taken at four waste water treatment plants in Maryland. The percentage of MRSA positive samples decreased as treatment within the waste water treatment plant progressed, and the MRSA bacteria did not survive in the plants that used chlorination, a tertiary step in wastewater treatment.

Today most of the Washington DC region’s waste water treatment plants use tertiary treatment and the waste water is treated to meet the stringent standards to protect the Potomac estuary and Chesapeake Bay where most of the regional waste water treatment plants discharge. Waste water treatment plants with tertiary treatment are called “advanced” waste water treatment plants, but that is not as modern as it sounds.

At waste water treatment plants primary treatment screens wastewater, and performs some rudimentary treatment to remove crude solids of human waste, all the trash flushed down toilets and skim off grease, oil and fat. Then wastewater sits in settling tanks, which are designed to hold the wastewater for several hours. During that time, most of the heavy solids fall to the bottom of the tank, where they become a thick slurry known as primary sludge. Primary Treatment consists of sedimentation and removal of large debris using screens and the large settling tanks. Until 1960’s primary treatment was the only form of sewage treatment in most sewage plants.

Secondary treatments usually includes biological and/or chemical treatment. One of the most common biological treatments is the activated sludge process; in which primary wastewater is mixed with bacteria that break down organic matter and cleans the water. Oxygen is pumped into the mixture. A clarifying tank allows sludge to settle to the bottom and then the treated wastewater moves on for tertiary treatment.

Coagulation, filtration and disinfection take place in tertiary treatment. A coagulant is added, the commonly used high-lime process can reduce phosphorus to below 0.10 mg/L. This process also serves as a barrier to viruses, captures organics leaving secondary treatment, and precipitates heavy metals and other suspended particles. Coagulation is followed by filtration which removes organic matter, microorganisms, minerals and excess nutrients. The final barrier to pathogens is a chlorination and dechlorination processes.

Since 1978, the upper Occoquan Sewage Authority (an advanced waste water treatment plant) has been discharging tertiary treated water into a stream above Occoquan Reservoir, one of the two potable water supply sources for Fairfax County, Virginia. Recycled water has been part of the Occoquan supply for 34 years and chances are if you are in Fairfax, parts of Prince William and Loudoun counties you have been regularly drinking recycled water. Fairfax Water fully treats and tests all water they send out as potable. The Occoquan Watershed Laboratory (OWL), operated by the Virginia Polytechnic Institute Department of Civil Engineering conducts comprehensive studies of the Occoquan water quality, and effects of the waste water treatment effluents on the watershed and water supply.

The Washington DC metropolitan area has 24 regional waste water treatment plants that treat 90% of the regions waste water. About 10% of the region is served by on-site septic systems. Combined, the waste water treatment plants treated an average of 544 million gallons of water a day in 2013. Blue Plains remains the largest plant in the region. The Washington DC region also has over 16,000 miles of pipes and pumping stations that move sewage from our homes to the waste water treatment plants. Due to the age of the Washington DC and Alexandria systems, parts of those systems are what is called combined systems where sewer and stormwater are carried through the same pipes. These systems tend to be overwhelmed by the volume of water during rain storms.

The Blue Plains Advanced Waste Water Treatment Plant is under a consent order from the Environmental Protection Agency, EPA, to meet new effluent limits for total nitrogen released and better control of the system during storms and has just completed boring a four-and-a-half-mile-long tunnel that will be used to store the excess storm and sewage water flow during rains and reduce releases of raw sewage into the Anacostia River. This tunnel is part of a $7.8 billion 20 year improvement program called the Clean Rivers Project.

In 2013 the Washington DC region spent about $1.3 billion on capital investments primarily at the waste water treatment plants and spend $950 in operating cost for their waste water systems. The majority of this money was spent at the central waste water treatment plants. Though the region’s waste water treatment plants have made sizable investment to meet the U.S. EPA discharge permit limits, less has been spent of the sewer piping system. The Metropolitan Washington Council of Governments that over $1 billion will need to be spent each year to replace and properly maintain the aging sewer pipes, pumps and valves in the waste water gathering system. As our aging sewers fail, public health and the water quality of our rivers, streams and estuaries are threatened.

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