A Submerged Attached Growth Bioreactor for Decentralized Wastewater Treatment

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Courtesy of Water Environment Federation (WEF)

ABSTRACT
The Amphidrome® process is a submerged attached growth bioreactor (SAGB) that has been used in Massachusetts, Connecticut, Rhode Island and New Jersey for decentralized wastewater treatment systems ranging in size from 440 gpd to 150,000 gpd. The two primary advantages of a SAGB are the small volume requirement and the elimination of downstream clarification. Five years of data from four plants operating in Massachusetts are presented below. Each facility was designed to treat a domestic wastewater to an effluent BOD5 ≤ 30 mg/l, TSS ≤ 30 mg/l, and total nitrogen ≤ 10 mg/l. The process is described and the performance, including loading and removal rates, at the four treatment plants is presented. The results indicate 1) 97% nitrification at an organic loading of 2.5 kg-BOD5/m3-day, and 2) a nitrification rate of 0.427 kg-N/m3-day and a denitrification rate of 0.410 kg-N/m3-day each at a total ammonia loading of 0.434 kg- N/m3-day.

INTRODUCTION
Due to the severe impact of eutrophication on water resources, the removal of inorganic nutrients, nitrogen and phosphorus, from wastewater has become an increasingly important consideration and has imposed new challenges in the design of wastewater treatment plants. Nitrogen discharge limits for many coastal regions and tidal estuaries have become especially stringent in recent years and biological nutrient removal (BNR) processes have been developed to meet the challenge. One such technology is the submerged attached growth bioreactor (SAGB), which derives its name from the fact that the media is always submerged in the process flow. The two primary advantages of a SAGB are the small volume requirement and the elimination of downstream clarification (Grady et al. 1999). A submerged biofilter allows for a high biomass concentration leading to a short hydraulic retention time and, thus, a significantly reduced reactor volume as compared to a different fixed film reactor or a suspended growth reactor. In addition, the media in a SAGB may be fine enough to provide physical filtration for solids separation.

During the last twenty years, different configurations of SAGBs have been conceived and advances in the understanding of these systems have been made. SAGBs have been used to achieve complete nitrogen removal by combining the aerobic oxidation of soluble organic matter (SOM) and nitrification into one operational unit and denitrification into a separate unit operation (Andersen et al. 1995 and Holbrook et al. 1998). A pilot study of a biological aerated filter (BAF), a specific type of SAGB, installed in series and downstream of a denitrification unit demonstrated that a recirculation of 300% of the inflow, from the filter back to the denitrification unit, removed organics and nitrogen to the required levels (Yoshinobu et al. 1997). Combined removal of organics and nitrogen was demonstrated in a single BAF with a separate anoxic zone created within the reactor (Rogalla and Bourbigot 1990). However, the use of a single-unit, single-zone SAGB for achieving the combined removal of organics and nitrogen is an innovative process variation.

THE SAGB PROCESS
This particular SAGB process was specifically designed for the combined oxidation of carbonaceous matter, nitrogen removal and suspended solids removal in a single-unit single-zone biofilter. The system includes one anoxic/equalization tank, one clear well and one SAGB. (See figure 1.) and operates as a sequencing batch reactor in which the wastewater is cycled back and forth through the filter. The biofilter is intermittently aerated to achieve both the aerobic environment required for the oxidation of organics and nitrification and the anoxic environment required for denitrification. It provides low visibility, since all tanks are typically installed underground, compact footprint, effective nutrient removal and minimal effect from cold air temperatures.

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