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WASHBURN TUNNEL WASTEWATER TREATMENT FACILITY

    The Washburn Tunnel Wastewater Treatment Facility (WTF) is a regional collection and treatment facility that treats the waste from: one municipality, nine industrial plants via pipeline, two industrial plant via barge and the WTF's trucked-in waste receiving facility known as the Vince Bayou Receiving Station (VBRS). The VBRS primarily receives chemical toilet  and septic wastes. However, Class I and Class II non-hazardous industrial wastewater is also accepted for treatment. WTF and VBRS are located in the Washburn Tunnel vicinity of the Houston Ship Channel in Pasadena, Texas on 35.633 acres of land. This publicly owned treatment works (POTW) began operations in May 1973 and was recognized as a first in the United States to mix industrial waste streams from different industries in a single biological joint wastewater treatment process. This 60-MGD facility employs a modified activated sludge process and, because of the composition of the waste streams, requires small amounts of pH control chemicals and low nutrient addition. The processes effectively remove more than 90 to 95 percent of the biochemical oxygen demand (BOD) and total suspended solids (TSS).

COLLECTION SYSTEM

    The waste streams from the plants and VBRS are pumped through force mains to the head of the system for primary treatment. Two of the industrial users transports its waste by barge to the treatment facility where it is off-loaded and pumped to the head of the system also. Each waste stream is continuously monitored for flow, pH and temperature. Continuous 24 hour flow proportional samples are collected seven days per week for regulatory requirements, customer billing and process control. Incoming waste streams are monitored continuously by plant operations personnel who are on duty 24-hours per day, 7-days per week.

PRIMARY TREATMENT

    Primary treatment consists of disinfection, pH control, coarse screening, grit removal, clarification, and sludge disposal. The combined waste stream passes through three-inch coarse bar racks, mechanically cleaned bar screens, and two (2) grit chambers.

    Four vertical propeller pumps, each with a capacity of 20,000 gpm, lift the wastewater from the grit chambers into two primary clarifiers. Each clarifier is 210 feet in diameter, with a 15-foot side water depth, has a cone shaped bottom with a mechanical rake and has a volumetric capacity of 3.8 MG. Detention time of each clarifier is eight hours under normal flow and a total of more than 900 tons of combined primary and bio-solids (8 to 12 percent) is removed on an average each day. The effluent from the primary clarifiers becomes the influent to the secondary system after the addition of micronutrient.

NUTRIENT ADDITION

    Microbiological activity requires the presence of micronutrient, i.e., nitrogen and phosphorous. Facilities are present for the addition of phosphoric acid and ammonia to the flow prior to its entry into the aeration basins. Excess ammonia (nitrogen), at times, is present in the refinery waste and treatment processes must be adjusted accordingly.

SECONDARY TREATMENT

    Secondary treatment consists of a cooling cycle (as required), an activated sludge treatment system with sludge recycle, clarification, and final discharge into the Houston Ship Channel. Some forward flow from primary treatment can be pumped through cooling towers for temperature control during hot weather, if necessary. Temperatures above 105° F can be detrimental to the biological population. The cooled wastewater is treated by biological stabilization in three aeration basins (160' x 250' x 15' deep) having a detention time of fourteen hours each under normal flow conditions. The oxygen demand is met in the aeration basins by twenty-four (24) 125 horsepower surface aerators, which aerate and mix the waste and biomass. By meeting this oxygen demand in the aeration basins, the oxygen demand of the wastewater, once it reaches the Houston Ship Channel, is reduced by more than 95 percent of the influent oxygen demand. The mixed liquor flows into six secondary clarifiers. Two of the clarifiers (200' diameter x 12' deep) are peripheral feed/peripheral overflow, and the other four clarifiers (100' square x 17.5' deep) are center feed/peripheral overflow. The settled bio-solids are either returned to the aeration basins or wasted for ultimate disposal by way of seven recycle pumps.

    The effluent is discharged via two 40-inch lines to the Houston Ship Channel, Segment No. 1007 of the San Jacinto River Basin. Laboratory analyses are conducted routinely for organic, inorganic, and heavy metal constituents to insure compliance with regulatory effluent standards.

SOLIDS DISPOSAL

    Solids are generated in the plant at three points: (1) grit - removed in the grit chamber and mixed with dewatered filter cake for final disposal; (2) primary - gross solids removed in primary clarifiers and pumped via primary sludge pumps directly to the sludge dewatering facility; (3) secondary - bio-solids generated in aeration basins and removed in final clarifiers. Wasted bio-solids (not returned to the aeration basins) are either sent back to the head of the plant where they are removed with the primary sludge or pumped into a sludge thickener and then directed to the sludge dewatering facility.

    Waste sludges processed at WTF are composed of a mixture of 30 to 40 percent (by weight) waste activated bio-solids and 60 to 70 percent primary industrial sludges. Industrial sludges are derived from petrochemical, refining, cogeneration, bulk storage terminals, chemical, and pulp and paper mill wastewaters. Sludge from the primary clarifiers is comprised of 8 to 12 percent solids and is withdrawn by four (4) sludge pumps. The sludge pumps are positive displacement plunger pumps, and with adjustable stroke and speed, achieve a maximum capacity of 350 GPM each at solids concentrations of up to 12 percent.

    The sludge is pumped through an 8-inch line to a surge tank at the sludge dewatering facility where hydroxylated ferric sulfate is added to abate release of H₂S in the belt press building. The sludge dewatering facility consists of four (4) Ashbrook-Simon-Hartley Winkle belt presses with a capacity of producing fifty (50) dry tons of dewatered cake per day. With the addition of polymer, the filter cake (average 33 % solids) is loaded into trucks via a conveyor belt system. Once in the trucks, the stabilized Class II non-hazardous industrial solids are hauled to an approved Sub Title "D" landfill in Brazoria County.