Green Energy, Less Power, Fewer Chemicals, Cleaner Water, Class A Biosolids: A Transformational Program at Chattanooga's MBEC With a Financial Payback

Introduction The Moccasin Bend Environmental Campus (MBEC) in Chattanooga treats wastewater from a combined sewer system and currently has capacity of up to 140 million gallons per day (MGD). The liquids train treatment processes include primary clarification, high purity oxygen activated sludge (HPOAS) basins, secondary clarification, and chlorine disinfection. The solids treatment train includes gravity thickening, anaerobic digestion of a small fraction (~10 percent) of the total solids generated through temperature-phased digesters, centrifuge dewatering and post-lime stabilization of all biosolids to produce Class B biosolids. MBEC generates 55 dry tons of biosolids per day that is hauled offsite and land applied. Energy Audit MBEC recently completed a comprehensive Energy Audit to determine ways that both energy consumption and chemical use at the facility could be reduced. The audit evaluated energy and chemical savings from three potential major improvements in addition to several minor ones. The major improvements are based on:

*Installing thermal hydrolysis (THP) to enhance the digestion process and produce more biogas;

*Achieving complete nitrification to avoid very large hypochlorite expenditures due to nitrite lock; and,

*Converting the plant to diffused aeration with blowers and fine bubble diffusers. First, THP was evaluated as an anaerobic digestion enhancement alternative due to limited digester capacity and tight footprint at MBEC. Adding THP to the existing digestion system allows all sludge to be processed through the anaerobic digestion system, cuts biosolids generated in half (~$1.5M annual savings in hauling costs), eliminates lime use (~$570K annual savings in lime costs), produces Class A biosolids and boosts biogas production. The analysis recommended renewable natural gas (RNG) as the most favorable use of the biogas, given the potential for revenue generation. The net biogas available for RNG production is approximately 570 scfm and could account for $4M in annual revenue for the City at current RIN rates. Operational cost savings in the solids train are shown in Figure 1. Nitrification is difficult to achieve under the current MBEC configuration in dry weather conditions due to the short retention time and low pH conditions that are endemic to HPOAS systems. Meeting the 15 mg/l monthly average permit limit is difficult. In addition, the low pH conditions of HPOAS-based treatment can inhibit the growth of nitrite oxidizing bacteria, leading to high concentrations of nitrite in the secondary effluent. Nitrite has a high chlorine demand is responsible for over 50% of the chlorine consumed at MBEC. The alternatives analysis found that the most effective means of meeting MBEC's nitrification permit is by using membrane aerated bioreactor (MABR) technology. Utilizing MABRs avoided nitrite production, as the nitrifiers were fixed on submerged membrane cassettes inside the anoxic zone of the existing system, which has a higher pH, and results in ammonia removal without nitrite formation. Projected effluent quality from the plant SUMO model is shown in Figure 2. In addition, the analysis found that the lowest lifecycle cost option for replacing the oxygen plant at MBEC was to install blowers and fine-bubble diffusers as part of a conversion to aeration. The change to aeration supports the nitrification MABR recommendation, as the MABR reduces the soluble COD entering the aeration zones (reducing demand) and the switch to aeration removes the low pH conditions, reducing the residual nitrite formation. The annual average energy consumption from secondary treatment after the changes will be ~7 million kWh annually, a roughly 65% drop from the nearly 20 million kWh used at present. Conclusion The improvements listed here for MBEC result in very large operational cost savings throughout the treatment plant. The estimated capital cost for the program are $136M. See Figures 3-5 for capital cost estimates for the solids train, liquid train, and the combined program, respectively. The major operational cost savings are estimated as:

*$4M in annual RIN revenue from RNG

*$2.2M annually in reduced sodium hypochlorite use

*$1.5M annually in reduced cake hauling costs

*$900k annually in secondary treatment power costs Note that a $570k reduction in lime addition costs are roughly balanced out by increased polymer use, increased maintenance and a small increase in power in the solids train. These savings total $8.6M annually. The end result of the savings is a rare outcome in the industry — a major capital improvement program with a financial payback. A simple payback analysis has the entire improvement program paying for itself within 15 years. Further, the payback for the improvements is not solely dependent on generating revenue from RIN sales. The estimated simple payback without RIN revenue is 30 years. When the true costs of a 'do nothing' option are also included ($25M to replace the end-of-life oxygen plant, and an alternate solution to meet increasing difficult ammonia limits and sludge disposal challenges) the true payback becomes even shorter. However, this is not a project pursued solely due to a favorable payback. The City will also be more reliably able to meet their NPDES permit, will replace aging assets, and will be generating both cleaner water and Class A biosolids