Pilot Test Comparison of Six Different Membrane Bioreactors

Membrane bioreactors (MBRs) are a relatively new wastewater treatment technology which promises exceptional treatment efficiency and a reduced footprint compared to conventional treatment process trains. MBRs may be particularly well suited to situations in which water recycling is required or desired including satellite reclamation (sewer mining). MBRs are quite simply an activated sludge process in which the conventional secondary clarifier is replaced by a membrane separation process (either microfiltration or ultrafiltration). The MBR can be operated either with or without primary clarification, but always requires fine screening (3 mm or smaller) to protect the membranes from abrasive and stringy waste components (hair in particular). Due to the presence of an absolute barrier for suspended solids, MBRs are able to maintain very high solids concentrations (8,000 to 20,000 mg/L) and solids retention times which allows for smaller aeration basins and high BOD removals. Since MBR effluent is micro- or ultra-filtration permeate, effluent suspended solids are typically near the detection limit and turbidities are typically less than 0.2 NTU. As with other membrane systems, the most important characteristics are the membrane flux and the membrane permeability both of which are highly temperature dependent (lowest temperature controls design). Flux is often expressed as gallons permeated per day per square foot of membrane area (GFD) and permeability is usually the clean water flux per unit transmembrane pressure (TMP). With correct process design, MBRs can accomplish the same things as any activated sludge process including BOD removal, nitrification, denitrification, and biological phosphorus removal. The City and County of Honolulu (Division of Wastewater Treatment and Board of Water Supply) have a series of wastewater treatment applications for which MBRs may be feasible. These include 1) treatment of raw wastewater at pump stations for nearby water recycling applications, 2) treatment of primary effluent to upgrade existing wastewater treatment plants for water recycling, 3) treatment of primary effluent for concurrent nitrogen and phosphorus removal for discharge in environmentally sensitive areas, and 4) treatment of a high-strength solids handling recycle stream for organic and color removal. Because there was no experience with MBRs in Hawaii, a side-by-side pilot study with five MBR vendors was conceived and is currently underway at the Honouliuli Wastewater Treatment Plant in Honolulu, Hawaii. The study is being conducted in two phases. In the first phase, three different waste streams were treated one at a time by the MBRs side-by-side: raw wastewater, high-strength solids handling recycling stream (centrate), and primary effluent. In the second phase, some of the MBRs will be moved to other locations for pilot testing associated with full-scale design and a county-wide MBR application feasibility study will be conducted. Under each operating condition in Phase I, the following parameters are being evaluated for each MBR wastewater treatment system: 1) basic performance, 2) operation and maintenance requirements, and 3) life cycle treatment costs. One of the key features of the study is that City operations staff are operating and maintaining the systems and keeping detailed records three shifts per day. This paper will describe the pilot testing procedure and some of the analytical results obtained.