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The Benefits of the SNDR to the City of Delphos ThermAer ATAD Treatment Process

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The City of Delphos utilizes Autothermal Thermophilic Aerobic Digestion (ATAD) or a ThermAer ATAD system supplied by Thermal Process Systems, Inc. for solids treatment. The Delphos facility is a newly constructed (2006) state-of-the-art facility which produces a Class A biosolids product from the ThermAer system. The reduced hydraulic retention (HRT) of the thermophilic reactor allows for a reduction in footprint or ability to, in the case of retrofits, increase capacity without adding new tankage. This system was chosen for design specifically for its ability to meet Class A criteria, its record of achieving at least 50% total solids reduction and 60 to 80% volatile solids reduction, and its lack of odor generation due to the aerobic nature of its design and the integral 2-stage ammonia scrubber/biofilter odor treatment system.

Under current normal operating conditions, thickened sludge is introduced to the fully enclosed reactor tanks. These tanks are operated in parallel. Although during start-up these tanks were operated in series due to questions that arose during construction regarding the volatile solids content of the Membrane Bioreactor (MBR) sludge and whether this would be high enough to bring the reactors up to a proper operating temperature. The reactors came to operating temperatures of 150 degrees F (65 C) within two weeks of start-up and within 4 weeks the temperature of reactor 2 was at 178 ° F (81 °C). At that time, the operations were switched back to the original design of a parallel operation and have remained there every since because it was apparent that the VS content of the MBR solids was enough to keep the operating temperatures of the tanks within design parameters. The average operating temperature of the two reactor tanks are now 140 – 155 °F (~60-70 °C).

The operating temperature of the tank begins to drop as the reactors are fed cool, thickened raw sludge material. Once the feed cycle ends the temperature immediately begins to rise as the thermophilic micro-organisms consume the volatile solid material. Air (oxygen) is continuously added to the process by positive displacement blowers through a jet aeration header system. These blowers are variable frequency drive (VFD) driven and adjust air delivery during treatment based on the oxidation reduction potential (ORP) signal being measured in the reactor.

From the ATAD reactors, the digested biosolids are transferred to the ATAD storage tank (storage nitrification/denitrification reactor or SNDR tank). The addition of this tank to the system is important as it provides a method of cooling biosolids to reduce potential odor generation and belt filter press (BFP) polymer consumption and reduces the ammonia concentration in the BFP filtrate which also reduces the aeration demands of the liquid treatment processes.

While in the SNDR tank, the biosolids are furthered cooled via the heat exchanger to approximately 95 degrees F (35 C); thus, the temperatures drop from a thermophilic range to a mesophilic range. The cooling of the biosolids is an important part of the odor control system, (re-uptake of the ammonia via the nitrification/denitrification process facilitated by cooler temperatures and lower pH) and acceptable solids dewatering. The greater than 50% TS destruction in the process had a significant impact on the project economics and was a major factor in the ultimate decision to utilize the TPS ThermAer ATAD system at Delphos saving the facility substantial transportation and disposal dollars.

This paper will focus on the operations, potential issues and related benefits of the SNDR tank to the entire ThermAer ATAD treatment process that have been recognized at the Delphos facility during the first three years of operations. It will provide data on the control mechanisms for the reactors, trending done during normal and excessive loadings, ammonia recycle rates and total and volatile solids destruction data across the entire system.
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Keywords: ATAD; Biosolids; Class A; MBR; Membrane Bioreactor; SNDR; ThermAer; ammonia; digestion

Document Type: Research Article

Publication date: 2010-01-01

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