Blue Plains Advanced Wastewater Treatment Plant (AWWTP) Upgrade, United States of America


Washington

Construction works are currently being carried out to further upgrade the Blue Plains Advanced Wastewater Treatment Plant (AWWTP) located in Washington DC.

The project is being implemented to meet the requirements of the National Pollutants Discharge Elimination System (NPDES), nutrient load reduction goals of the Chesapeake Bay Agreement (CBA) and seasonal nitrogen and phosphorus effluent discharge limits.

The upgradation activities, scheduled for completion in 2015, will primarily witness the inclusion of a new biosolids processing main process train (MPT) incorporating the Cambi's thermal hydrolysis process (THP) for the treatment of biosolids. It will be the first of its kind facility in the country and the largest of its kind facility in the world to implement this technology.

Current upgradation works commenced in May 2011. The District of Columbia Water and Sewer Authority (DC Water) is the owner of the plant, with CDM Smith and PC Construction as the main contractors.

The total investment for upgrading the plant is expected to reach $400m. The design allows for future expansion to include four more mesophilic digesters, additional centrifuges and Cambi trains.

Details of the advanced wastewater treatment plant (AWWTP)

"The design allows for future expansion to include four more mesophilic digesters, additional centrifuges and Cambi trains."

The Blue Plains Advanced Wastewater Treatment Plant (AWWTP) was constructed in the 1930s. The plant was initially managed by the Washington DC Government. The District of Columbia Water and Sewer Authority (DC Water) took over the ownership of the plant in 1996.

The plant covers an area of about 153 acres with a normal capacity for treating 370 million gallons of wastewater per day and a peak capacity of one billion gallons per day.

The plant generates about 1,200t of biosolids for use in agricultural fields. The discharges from the plant are emptied into the Potomac estuary, which links with the Chesapeake Bay.

Monitoring of activities at the plant site are carried out through the central control room (CCR) and three additional area control centres (ACC).

Details of the Blue Plains treatment plant upgrade programme

The upgrade of the plant involves implementation of the Cambi's thermal hydrolysis process (THP) for the treatment of biosolids, and construction of a combined heat and power (CHP) plant with a capacity of 13MW to power the treatment plant. The expansion works will cover an area of six acres.

"The Blue Plains Advanced Wastewater Treatment Plant (AWWTP) was constructed in the 1930s."

Other construction works include the installation of facilities for biosolids blending and screening, pre-dewatering and four digesters with a capacity of 3.8 million gallons each. The nitrification / dinitrification facility, pump station, post aeration tanks, mixers and sluice gates at the plant site are also being upgraded as part of the project.

Upgradation works are also being carried out to enhance the nitrogen removal facilities. The works include upgradation of the slurry walls, flood wall, tanks and demolition of the existing lime building.

A three-storey building to enhance the capacity of dewatering Class A thermally hydrolysed-digested biosolids will be constructed. The building will incorporate an additional 16 new Belt Filter Presses.

The project is expected to utilise 8,400t of rebar, 80,000 cubic yards of concrete and include the placement and compaction of about 105,000 cubic yards of excavation and backfill.

Benefits of the Blue Plains AWWTP upgrade

The existing plant currently produces Class B lime stabilised biosolids using lime. The current project will upgrade the existing process and enable it to produce Class A biosolids.

The technology involved will cut down shipping of biosolids by 50%, reduce the use of energy from the grid by 30% and cut down emissions from the plant by 41%.

The project will save Washington D.C. and the neighbouring areas about $10m in power savings and $10m in reduced sludge disposal costs on an annual basis.

Treatment process employed at Blue Plains water treatment plant

The first step of the treatment involves the removal of grit and debris, after which the water is allowed to flow into primary sedimentation tanks where half of the sludge is removed.

The untreated water is then allowed to flow into a secondary tank where organic matter is allowed to break down by pumping oxygen into the tank. Microbes convert ammonia into harmless nitrogen gas in the next step.

Residual solids after these steps are removed and the water is then filtered using sand filters that remove phosphorus and the remaining solids. The water is then disinfected and dechlorinated before being discharged into the Potomac River.

Sludge from the primary sedimentation tanks is sent to tanks where it is allowed to settle down at the bottom and thickened through gravity. Sludge from the secondary and nitrification reactors on the other hand is thickened using flotation thickeners.

The thickened sludge from all the tanks is dewatered and finally pathogens are removed using lime before being sent to agricultural fields.

Details of the Cambi Thermal Hydrolysis Process (THP)

Thermal Hydrolysis Process (THP) is a digestion pretreatment step that uses heat and pressure to kill pathogens in the biosolids and break down biosolids so that they become more easily digested. The process also helps in cutting down odour problems during the treatment of organic materials.

The process involves the dewatering of sludge to dry solids by about 15%, which are then transferred to a storage tower. The dewatered sludge is sent to a pulper and then mixed and heated using recycled steam from the reactors and flash tank. The processed gases are then biologically broken down in digesters.

Thermal hydrolysis is allowed to take place in reactors at temperatures ranging from 165ºC to 170ºC for about 20 minutes. The steam is released and sent back to the pulper.

Sterilised sludge is flushed into the flash tank using a residual pressure in the reactor where the residual steam is flushed off and the steam recycled to the pulper after reducing the temperature of the sludge using heat exchangers and diluted water.

The THP process later goes through anaerobic digestion where the organic sludge is converted into biogas. The digested sludge is finally dewatered into biosolids by about 40% and used in agricultural land.

The contract to provide construction management services for the nitrogen removal facilities was awarded to Aecom. The five-year contract, which was awarded in March 2011, is worth $40m.

Water project

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