MEMBRANE BIOLOGICAL REACTORS TO REMOVE NITRATE, DIGEST BIOSOLIDS, AND ELIMINATE WATER FLUSHING REQUIREMENTS WITHIN REPLICATED RECIRCULATION SYSTEMS CULTURING RAINBOW TROUT

Nutrients, particularly nitrate (NO₃), can accumulate to very high levels within low exchange recirculation aquaculture systems (RAS) and negatively impact a number of cultured species. To prevent the harmful effects of nitrate accumulation and to dispose of concentrated waste biosolids, many RAS are operated with makeup water as high as 0.5-5% of the total recirculating flow. This means that a fish farm capable of producing 1,000 mt/yr requires a minimum of 800 m³ of daily makeup water even with relatively low water usage; most such farms would operate with at least twice this makeup flow. While RAS designs use much less water than typical flow-through fish farms, these water requirements and the associated water withdrawal and point-source discharge permits can still limit RAS technology implementation.

Membrane biological reactors (MBRs) are a promising and scalable technology that could allow complete closure of the RAS, decreasing both the water withdrawal and point-source discharge requirements. MBRs have been widely used to remove most of the nutrients, solids, and organics from municipal, industrial, and agricultural wastewaters. Using a series of membranes plumbed in parallel, an MBR operates with both anoxic and aerobic stages within an activated sludge so that complete nitrification and denitrification can occur. In previous research, we found this technology effective at removing solids and nutrients from RAS effluent while aerobically stabilizing the system waste into non-malodorous biosolids suitable for composting or land application as a nutrient-rich soil amendment. The MBR removed nearly all of the nitrogen (< 3 mg/L), phosphorus (< 0.1 mg/L), biochemical oxygen demand (< 1 mg/L), suspended solids (< 1 mg/L), and heavy metals from RAS wastewater. Given these low nutrient levels, it follows that the MBR's permeate is likely well-suited for reuse in a fish culture system. Because the bacterial biomass in biosolids accumulates during MBR treatment, some of this biomass must be removed periodically in a process termed "biosolids wasting." As a result of the requirement to remove these biosolids, we estimate that MBR's could allow for a nearly 10-fold reduction in water flushing requirement over the already tight water recirculation loops in many RAS.

In the present study, six replicated RAS are being used to determine if an MBR can be integrated within a RAS to control nitrate accumulations at levels < 100 mg/L, while simultaneously treating all of the biosolids backwashed or flushed from the primary solids capture processes in the system. This research will  also determine if these MBR-containing RAS that operate with almost no makeup water flushing can sustain salmonid growth, welfare, and product quality.