Ammonia reduction and nitrification5 March 2002
Garden State Tanning's Williamsport plant (GST) is completely isolated from the regional (Washington County, Maryland) wastewater collection system and, therefore, the tannery's effluent treatment plant treats 100% of its sanitary and industrial wastewater. On average the wastewater treatment plant (WWTP) processes 0.5 MGD and disposes of approximately 900 wet tons of sludge per month. The tannery's effluent treatment process has evolved from the plant's inception in the 1880s using earthen lagoons in a four phase process totally contained system. The closure was unique in that waste cement kiln dust was used to solidify the lagoon which at some points was 25ft in depth. The 3.5 acres were closed following toxicity testing at a cost of $1.2 million. This expense is low in comparison with other closures and surpasses all the state and federal requirements due to the kiln dust closure. In 1990, a two million gallon biological reactor was placed into service to provide secondary biological treatment. The reactor would not convert ammoniacal nitrogen during the winter months because the mixed liquor temperatures would become too cold to support the biological nitrification process. Since nitrification requires an optimal temperature above 8°C, GST installed a floating insulating cover and aerator hoods over the entire surface of the biological reactor which prevented the mixed liquor temperature from dropping too low to support nitrification (Figure 1). Although the original NPDES Permit established winter/summer limits, toxic reduction ammonia and nitrogen reduction pressures from the Chesapeake Bay required year round nitrification. Additionally, a belt filter press was installed adjacent to the biological reactor to press biological sludge from the secondary clarifier and maintain a consistent mixed liquor concentration. Tighter controls were placed on maintaining a longer sludge age during the winter months to help establish nitrification. The sludge age is now 12 to 15 days in the summer and 25 to 30 days in the winter. The winter adjustment allows more residence time for treatment with a higher MLSS. However, the reactor still did not nitrify during the winter of 1998-99. Nitrification also requires constant alkalinity supply, therefore a steady and constant beamhouse schedule was established to normalise this parameter. In the spring of 1999, GST commissioned an evaluation of their complete WWTP, focusing on the pre and primary treatment phases of the process. As a result of that evaluation, a primary treatment process flow change to enhance sludge settling was recommended, along with sludge collection and de-watering improvements in the primary treatment area. A target of 70% removal of solids (BOD) was set for the efficiency of the primary treatment phase. A primary clarifier was converted to an equalisation tank. Equalised retan/paddle wastewater was mixed with oxidised sulfide prior to primary clarification. Aeration was doubled because sulfide oxidation tanks were used for wastewater equalisation. Coagulant and flocculate dosing systems were installed and primary sludge de-watering capacity was enlarged and efficiency improved. These changes also resulted in a cost reduction with less coagulant required. Primary clarification was rebuilt to ensure constant sludge removal. New inlet baffling was added to ensure constant sludge removal. The results were equalised flow with controlled conditioning prior to clarification resulting in improved BOD5 and solids removal. TSS removal averages 75% along with a 60% BOD removal. A series of improvements were also made to the secondary biological treatment system. A preventative maintenance programme was established for the lubrication schedule, which allowed for increased aeration. This gain in aeration resulted from additional aeration discs, less down time for repairs, and increased submergence of the discs to improve surface capacity. These low cost improvements were only possible because of the preventative maintenance programme. Improvements were also made to the secondary clarifier to improve control of the returned activated sludge. This is another low cost improvement but a large gain in treatment capability. The improved control of clarifier sludge depth also prevents the formation of sludge blankets. This can create anaerobic conditions which produce facultative anaerobes due to the production of toxins which kill off the nitrifiers. Now that the solids had been removed, efficient sludge treatment systems were needed. The operation of a separate belt press handles the biological sludge and with it GST are able to maintain their correct sludge age. But their filter presses were slow and inefficient in water removal. The first improvement was to only treat primary sludge in the presses which reduced plugging and obstruction of the cloths. By selecting cloths with greater porosity and increasing the pump pressures, GST were able to cut the cycle time in half (to six hours) and decrease the water content by 50%. These gains allow for increased capacity and decreased landfill costs. Biological considerations also contributed to an improved treatment system with increased capacity. The biologic population was reviewed for metabolic activity, settling characteristics for the heterotrophs and the stability under stressful conditions for the nitrifiers. Biologic SB-1 was chosen due to its high metabolic and replication rate. This is suitable for a tannery environment where the conditions are variable and re-application is required. A secondary benefit was realised due to the sticky polysaccharide coats and bio-polymer secretions of the protozoan population (stalked ciliates) which allows for the solids and bacteria to adhere. The expensive nitrifiers, which are usually washed out in the clarifier, are recycled through the sludge return due to the increased adhesion. This has reduced costs through less nitrifiers being required and a large decrease in polymer addition. The natural flocculation made an improvement in the sludge settleability. A new conditioner which is an extract of peat moss is now used to aid the biological population. The extract stabilises nitrification, boosts metabolic rates and decreases sludge generation. During the winters of 1999-2000 and 2000-2001, the biological reactor nitrified to completion for year 2000 daily average concentration of 0.69mg/l of ammonia in the effluent down from an average concentration of 119.4mg/l in 1998. The WWTP discharged 52,000lb of total nitrogen to the Potomac River per year in 1998 contrasted to 21,400lb in 2000 for a 58.9% decrease. Primary treatment BOD removal increased by 39.7% by 2000 from an estimated 1998 BOD removal of only 30%. The cost of WWTP operations decreased in 2000 and continues to drop. Overall, the treatment costs were significantly below budget with the new processes. The savings are in chemical addition since GST have reduced their polymer usage. Additionally, to prevent pollution of the Potomac River, secondary containments were constructed around WWTP bulk chemical storage areas. High liquid level alarms were installed on chemical storage tanks. Sludge dumpster and chemical storage areas were installed with sloping surfaces and raised perimeters to channel leachate and spilled chemicals to the WWTP were built. The storm water retention basin was also improved. A new WWTP laboratory was constructed to insure that proper, consistent and frequent process control was achievable. All these improvements, even though they appear small, went right to the bottom line in savings. Most of the changes are not expensive and improved both the treatment and reduced costs. With the savings noted above, these investments are already paying off. Finally, in order to strive for excellence, GST sought and received ISO 14001 certification in March 2000 and certification was renewed in March of 2001 making Garden State Tanning the first and only US tannery with this certification for continuous improvement.