Integrating membrane biological reactors with RAS

Authors: DAVIDSON, JOHN - Freshwater Institute; SUMMERFELT, STEVE - Freshwater Institute; VINVI, BRIAN - Freshwater Institute; Schrader, Kevin; GOOD, CHRISTOPHER - Freshwater Institute

Submitted to: Hatchery International Magazine
Publication Type: Trade Journal
Publication Acceptance Date: 3/1/2019
Publication Date: 6/7/2019
Citation: Davidson, J., Summerfelt, S., Vinvi, B., Schrader, K., Good, C. 2019. Integrating membrane biological reactors with RAS. Hatchery International Magazine. May/June 2019, pages 28-29.

Interpretive Summary:

Technical Abstract: Membrane biological reactors (MBRs) are widely used for municipal, industrial, and agricultural wastewater treatment to remove nutrients, organics, and solids from concentrated effluents. MBRs utilize fine-pore membranes that create a clean, low-solids filtrate, while associated aerobic and anoxic processes functioning within an activated sludge system facilitate nitrification and denitrification, respectively. Onsite research has shown that MBRs are a promising wastewater treatment technology for aquaculture effluents. However, this technology may also be well-suited for integration in the water recycle loop of RAS. A four-month study was carried out to evaluate the feasibility of incorporating MBRs within RAS. Six replicate RAS (9.5 m3) were used for this research. Three RAS included MBRs and only received new water to replace evaporative loss, splashing, and minor system overflows. The other three RAS were operated without MBRs and utilized standard flushing rates known to provide conditions for acceptable rainbow trout health and performance. Replicate RAS were stocked with equal numbers of rainbow trout (103 ± 1 g) prior to beginning the study. RAS with MBRs used six and a half times less water than RAS without MBRs. Mean system hydraulic retention time for RAS with and without MBRs was 104 ± 31 and 13 ± 1 days, respectively. Backwash solids did not provide enough carbon to completely drive denitrification; therefore, a small amount of sugar was periodically added to MBRs. Nitrate-nitrogen, total ammonia-nitrogen, total phosphorus, true color and dissolved calcium, copper, magnesium, and sulfur levels were greater in RAS with MBRS, while alkalinity and ultraviolet transmittance were statistically lower (P< 0.05). Nevertheless, rainbow trout performance was generally unaffected by the different culture environment. At study’s end, mean rainbow trout weight in RAS with and without MBRs was 595 ± 14 and 623 ± 6 g, respectively (P > 0.05). Common off-flavor (geosmin and 2-methylisoborneol) levels measured in fish flesh and RAS culture water were not significantly different between treatments. Integrating MBRs within RAS resulted in substantial water savings and was biologically feasible for rainbow trout production but requires further optimization. MBR permeate was produced at a slower rate than specified in the design, resulting in periodic system overflows. If acceptable MBR permeate flows can be maintained, a range of improvements could result including increased water savings, greater water exchange through the MBR, and enhanced treatment capacity to reduce nitrate levels and concentrations of other accumulating compounds in the culture water. Ozone could also be utilized to optimize the culture environment of MBR-integrated RAS.