Evaluation of geotextile filtration applying coagulant and flocculant amendments for aquaculture biosolids dewatering and phosphorus removal

Authors: SHARRER, MARK - Freshwater Institute; RISHEL, KATA - Freshwater Institute; SUMMERFELT, STEVEN - Freshwater Institute

Submitted to: Aquacultural Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/3/2008
Publication Date: 1/1/2009
Citation: Sharrer, M.J., Rishel, K., Summerfelt, S.T. 2009. Evaluation of geotextile filtration applying coagulant and flocculant amendments for aquaculture biosolids dewatering and phosphorus removal. Aquacultural Engineering. 40:1-10.

Interpretive Summary: Waste generated in fish culture systems results from uneaten feed, fish feces, and biological material sloughed from surfaces and vessels. Mechanisms used to separate waste from fish culture water include gravity separation, mechanical filtration, and granular media filtration. Flushing or backwashing of these mechanisms results in a relatively concentrated flow of wastewater that requires further dewatering prior to removal. An onsite method to decrease fish culture waste volume can result in a reduction in costs associated with composting,incineration, off-site hauling, or land application. Geotextile bag filters are a highly scalable solids dewatering device that have the potential to provide a simple, low cost wastewater management system for fish farmers. Geotextile fabric is a highly durable, porous material that can be fashioned into bags and loaded with solids -laden water for dewatering purposes. Geotextile bag filters have been successfully used to dewater dredge slurry and agricultural waste lagoons resulting in a relatively clean filtrate. Typically, a polymer is added to the wastewater prior to bag loading, which aids in the formation of larger, aggregated solids that are more readily retained by the geotextile material. In addition, chemical coagulants used in wastewater treatment applications can facilitate precipitation of dissolved phosphorus. In this particular application, the precipitate is potentially retained within the geotextile bag and excluded from filtrate flow. The research objective was to evaluate geotextile bags for the capture, dewatering, and storage (for an approximately three month period) of waste collected from intensive fish culture systems equipped with gravity and mechanical separation techniques. Geotextile bag loading of polymer amended wastewater flow was evaluated in terms of the reduction of suspended solids and dissolved wastewater constituents in the geotextile bag filtrate. Secondly, a comparison of three chemical coagulants (aluminum sulfate, ferric chloride, and hydrated lime) commonly used in the wastewater treatment industry were evaluated to determine their effect on phosphorus and carbonaceous biological oxygen demand leaching in geotextile bag filtrate. Geotextile bag filters were determined to consistently remove approximately 95% of the suspended solids contained in fish culture backwash flows when amended with polymer plus alum, ferric chloride, or hydrated lime. Treatment efficiency of particulate forms of nitrogen captured along with the solids inside the bags was moderate applying ferric chloride (46.7%) and alum (39.1%). However, poor treatment efficiency of particulate nitrogen was evident applying hydrated lime as a coagulant (-8.9%). Similarly, particulate phosphorus treatment efficiency was moderate applying alum (67.6%), ferric chloride (47.0%), and hydrated lime (77.3%). However, as the captured biosolids aged within the geotextile bag filters, elevated concentrations of inorganic nitrogen and dissolved phosphorus leached into the filtrate. The significant amount of dissolved nitrogen and phosphorus as well as carbonaceous biological oxygen demand that leached into the filtrate is a disadvantage that must be considered before applying geotextile bag filters in typical dewatering and effluent treatment applications. However, geotextile bag filters could also provide an excellent pretreatment in an application where the solids must be dewatered for disposal, but leaching of inorganic nitrogen and dissolved phosphorus from these biosolids is desired to feed nutrients to a downstream hydroponic/aquaponic operation or used as irrigation for field crops. Geotextile bag filters also provide good solids dewatering, producing 19-22% biosolids concentrations 7-10 days after wastewater treatment additions were discontinued. As a result, cost associated with handling a

Technical Abstract: Wastes contained in the microscreen backwash discharged from intensive recirculating aquaculture systems were removed and dewatered in simple geotextile bag filters. Three chemical coagulation aids, (aluminum sulfate (alum), ferric chloride, and calcium hydroxide (hydrated lime)), were tested in combination with a long-chain polymer flocculation aid (HyChem CE 1950 at 25 mg/L) to determine the most cost effective and efficient treatment combination. Three different coagulants were tested to determine if coagulant choice impacts nutrient and carbonaceous biochemical oxygen demand (cBOD5) leaching into the filtrate and the final composition of the bag-captured biosolids at the end of each period. If nutrient leaching into the bag filtrate could be minimized through coagulant selection, then geotextile bags could provide a convenient and effective method to dewater waste biosolids and provide them in a form that fish farmers could readily transport, store, or send for disposal. Results from replicate geotextile bag filter tests indicate that when alum, ferric chloride, and hydrated lime (plus a polymer) were amended to a backwash flow, both suspended solids capture and solids thickening were improved; i.e., total suspended solids removal rates of 95.8, 95.1, and 96.0%, respectively, were achieved along with final dewatered filter cake percent solids concentrations of 22.1, 19.3, and 20.9%, respectively. Alum, ferric chloride, and hydrated lime (plus a polymer) amended geotextile bags were not as effective in chemical oxygen demand (COD) and cBOD5 removal, resulting in removal rates of 69.6, 67.2, and 35.3%, respectively, and 56.6, 9.3, and -47.4%, respectively. Further, the use of lime as a coagulant resulted in filtrate COD and cBOD5 concentrations that exceeded inlet concentrations. Total nitrogen removal applying alum, ferric chloride, and lime were also less than effective, resulting in removal rates of 39.1 , 46.7, and -8.9%, respectively. Filtrate total nitrogen concentrations were primarily in the inorganic form (total ammonia nitrogen) suggesting mineralization of ammonia as solids were stored within geotextile bags under anaerobic conditions. Alum, ferric chloride, and lime amended bags were moderately efficient at total phosphorus removal, resulting in removal rates of 67.6, 47.0, and 77.3%, respectively. Alum was identified as the most cost effective chemical for coagulation, but hydrated lime was the most effective at dissolved phosphorus precipitation and removal.