The city of Chattanooga is seeking Requests for Proposals for a major overhaul of the outmoded Moccasin Bend Sewage Treatment Plant, which serves some 400,000 residents over 200 square miles. Customers include 80 industries and nine regional users.
The aim is to install a Green Energy Program at the facility that currently processes 140 million gallons per day and is slated to process 210 million gallons per day by the end of 2028 when the plant overhaul is due to be completed.
The city is seeking a firm that will design the improvements and construct them (design-build), then the city will take over operation of the improvements.
The project is funded from a combination of sources. The city secured a loan from TDEC’s Clean Water State Revolving Fund (CWSRF) program, and a Water Infrastructure Finance and Innovation Act (WIFIA) loan from the USUSEPA. The remaining funding will be provided from the city's Enterprise Sewer Fund.
The city plans to design and construct improvements to anaerobic digestion, nitrification, and aeration processes at the facility.
Officials said:
Issues to be addressed by the Project Aeration MBEC creates oxygen for the secondary treatment process with an aging Cryogenic Oxygen Production Plant. The high purity oxygen (HPO) from the facility is then supplied to the secondary treatment (UNOX) tanks where the oxygen is transferred into the process water and consumed by the mixed liquor.
The Oxygen Plant has reached its end of life and failed for 2 weeks in June 2023.
Fifty percent of the oxygen produced is lost via leaks.
The oxygenation system for secondary treatment is by far the largest power consumer at the plant. The oxygen plant utilizes 13.7 million kWh/year. The UNOX basins also have 32 surface aerators to transfer the oxygen into the process water. These surface aerators use an additional six million kWh/year for a total energy use in secondary treatment of 19.7 million kWh/year. This energy use has had an average annual power cost of $1.3 million per year.
Nitrification MBEC currently has an effluent ammonia NPDES permit limit of 15 mg/L monthly average, 20 mg/L weekly average, and 30 mg/L daily maximum. Currently, MBEC struggles to meet the monthly limit with the UNOX system under various conditions.
Winter: when the temperature drops, MBEC is unable to remove the influent ammonia to consistently meet the 15 mg/L permit due to decreased biological activity in cold temperatures. The UNOX system’s volume is insufficient to maintain a nitrifying SRT in cold weather. MBEC can meet the ammonia permit with the help of rainwater dilution at the plant. If there is a significant stretch of dry weather during the winter, MBEC would exceed the effluent ammonia permit limit.
As new development in Chattanooga introduces low flow systems such as high efficiency faucets and low flush toilets, and reduced infiltration and inflow, the dry weather conditions will continue to be more frequent and a bigger issue for effluent ammonia compliance.
Summer: with the UNOX system, the CO2 produced during the process is unable to release into the atmosphere and the plant has issues achieving full nitrification. When the CO2 remains dissolved in the wastewater (i.e., production of carbonic acid), the wastewater pH is lowered. At low pH, nitrification can be inhibited, and in the case of MBEC, only partial nitrification is achieved, converting ammonia to nitrite. Although the ammonia can reach limits below the effluent permit, partial nitrification leads to other issues for the plant, particularly in the disinfection process.
It is a well-known phenomenon that nitrite chemically reacts with chlorine (at a 5:1 ratio), and therefore, any nitrite that enters the disinfection process uses up significant amounts of the chlorine dose. The nitrite in the disinfection process leads to extremely high chlorine doses and rapidly changing doses. Chlorine doses that are high and rapidly fluctuating makes disinfection expensive for the plant (more than $4 million per year) as well as lead to operational difficulties; if the dose is not updated quickly enough, the plant could see permit violations. The effluent ammonia decreases when it is converted to nitrite via partial denitrification, but when nitrite concentrations increase, the chlorine demand increases as well. The MBEC has six anaerobic digesters.
The sludge produced at MBEC is only partly digested. Part of the primary sludge is sent through gravity thickening before thermophilic anaerobic digestion, and then followed by mesophilic digestion (thermophilic digestion volume of 1.1 MG at 55°C and mesophilic digestion volume of 2.3 MG at 35°C). The Waste Activated Sludge is not digested at all and currently there is not an efficient method to pump waste activated sludge to the digesters.
Equipment in the digesters such as valves, pumps, instrumentation, electrical systems, mixing system, and boilers are heavily worn. A thorough investigation of this facility will need to be done.
The digestion process produces flared biogas consisting primarily of methane and carbon dioxide. There is currently no reuse of biogas to produce green energy.
There is a limited footprint available onsite for building additional digesters.
The current solids treatment process involves labor intensive and costly use of lime ($600,000 per year).
The biosolids are only treated to Class B.
Approximately 15 trucks haul the biosolids daily to long-distance farms for land application. MBEC has been receiving complaints for odor, and the local population have rising concerns in terms of contamination and public safety.
Officials listed these Project Goals:
Produce a Class A Biosolid product and enhance the generation of biogas through digestion.
Aeration
Project should replace the failing secondary treatment aeration system as a priority. Nitrification 1. Project should allow the Owner to meet the NPDES permit and resolve existing nitrification issues while addressing ammonia from THP centrate.
Solids Treatment
Project shall produce Class A biosolids as per TDEC certification criteria.
Project shall enhance the production of Green Energy from anaerobic digestion and make a significant contribution to the City’s Climate Action Plan. 3
Project shall significantly reduce the volume of biosolids.
Project should eliminate the use of lime.
Project should include a plan for contingency for future EPA regulations on Per- and Polyfluorinated Substances (PFAS).
Project should include a plan for contingency for more stringent nutrient NPDES permit limits in the future.
Project should include a plan for contingency for future digestion of Fats, Oils and Grease (FOG), food waste, municipal green waste.
If THP is presented as an alternative, then the Project must include a plan for addressing the increased Ammonia from the THP centrate.
Project Scope Conversion to Aeration System
Add subsurface diffusers to supply air and achieve the needed dissolved oxygen. Panel diffusers may be used to maximize diffuser floor coverage. Fine bubble aerators are strongly preferred.
Add new blowers located within the existing blower building to supply the air to the diffusers. The required air flow may be supplied with single stage centrifugal blowers.
In addition to blowers and diffusers, add a reaeration process downstream of disinfection to comply with the effluent dissolved oxygen permit. MBEC’s effluent is required to have a minimum dissolved oxygen concentration of 4.0 mg/L. WWTPs with diffused air generally have lower dissolved oxygen concentrations than HPO facilities. As a result, it is customary for diffused air facilities to have a small reaeration process downstream of disinfection to add additional oxygen into the plant effluent. Adding oxygen downstream of disinfection is not very energy intensive, as the disinfected effluent has little to no bacteria demanding oxygen.
If the design team determines that conversion to aeration is a best value alternative, then abandonment and removal of the existing Oxygen Plant shall be included as part of the project.
Nitrification: Membrane Aerated Biofilm Reactor (MABR)
Install MABR cassettes in each of the UNOX. MABRs are a technology solution for plants that need to increase capacity (especially for nitrification and nutrient removal) but may not want to invest in additional tank space. With MABR technology, cassettes of hollow fiber gas permeable membranes are inserted into a portion of the existing aeration basin. The fibers provide a surface for the nitrification bacteria to grow in denser communities and have a longer SRT, increasing the amount of nitrification in the same amount of space.
Solids Treatment THP Option a.
Thermal Hydrolysis Pretreatment (THP) combined with mesophilic anaerobic digestion (MAD) is an identified digestion enhancement alternative due to limited digester capacity and tight footprint at MBEC.
The improvements may generally include a new wet well, new prescreening (possibly in exiting centrifuge building 1), a storage tank, predewatering (possibly reuse of existing centrifuges), new cake hopper, anaerobic digestion (the existing six digesters shall have capacity to process all solids generated at MBEC, however, one new in-kind digester may be needed to process future max month loadings), existing sludge storage tank, post-dewatering (possibly belt filter press or centrifuges in the existing filter press building).
Project will include the rehab of the existing six digesters.
Project will allow for the reuse of some of the biogas for other plant processes, and heating of the digesters.
Project will include the scrubbing of remaining biogas for reuse by a gas company (ready for pipe injection - gas management will be included as scope in separate contract and not part of this scope.)
City officials said they are opening to other treatment options "in a desire to find the most efficient and sustainable methods for replacing the current secondary treatment aeration system, resolving nitrification issues, achieving Class A biosolids, enhancing Green Energy output, and addressing various other aspects of our wastewater treatment process."