Ways to sustainably manage wastewater in RAS
April 8, 2021 By David Kuhn
Implementing recirculating aquaculture systems (RAS) to raise fish and shellfish is an excellent strategy for maximizing water reuse while minimizing wastewater discharge.
The first thing to understand is how RAS and water quality are managed. Since fish and shellfish that are cultured in RAS live in their own waste, the water needs to be consistently and constantly filtered to improve water quality. Good water quality is critical for animal welfare, health and production. Unsuitable water quality can be detrimental to the animals and the bottom-line in any aquaculture venture.
There are countless ways to design and operate a RAS successfully but they all have the same general unit operations. Solids filtration is used to remove suspended solids, uneaten feed and fecal material, from the water column. Examples include settling basins, radial flow separators, filter socks, centrifuges, bead filters and rotary drum filters. For the proteins and the smallest of suspended particles, these can be removed using protein skimmers. The efficiency of this foam fractionation process proportionately increases with the level of salinity or salt in the water. In addition to fecal material, fish and shellfish also excrete urine, other metabolites, and molecules that contain phosphorus. Toxic ammonia is converted to less toxic nitrate using biological filters such as moving bed biological reactors, trickling filters or other process units such as bead filters that contain a large surface area for beneficial nitrifying bacteria biofilms to form. Lastly, aquatic animals respire. The resulting carbon dioxide needs to be stripped from the water and oxygen is simultaneously added back using aeration, water agitation, or direct oxygen delivery.
It is critical for any aquaculture facility to have a properly designed and operated RAS to ensure healthy animal production while providing a conditions that is conducive to a healthy wastewater management plan.
The goal of RAS operators is to minimize water reuse by approaching zero water discharge. As a system approaches zero discharge, there are several waste products that accumulate and end up being the driving forces for water renewal, replacing a volume of water discharged from the system with new water. The two primary driving forces for water renewal are the accumulation of solids in solid filters and nitrate in the water column.
Solids will accumulate during normal RAS operations in solid filters. Each filter has a unique limit of how much accumulated solids it can handle. Filters that use settling as a mechanism will experience an increase in solids or sludge in the bottom of the tank. If the level of solids gets too high, the efficiency of the settling process will decrease significantly and toxic gases, such as methane, can be emitted from anoxic zones within the sludge into the water column.
Accumulated solids also have to be removed from bead filters using backwash cycles and scraped off the screen of the rotary drum as part of their routine operation and maintenance. Regardless of filter type, solids need be discharged from the RAS on a regular basis. Since these solids contain a notable amount of water, the operator is also discharging wastewater.
Nitrate accumulates in RAS as the biological filter cannot oxidize nitrate any further. It is normal to see nitrate concentrations in orders of magnitude higher than the more toxic forms of nitrogen (i.e. ammonia and nitrite) in RAS water. Nitrate toxicity varies greatly across aquatic species of fish and shellfish. For example, some marine species of finfish might be negatively impacted by 30 mg/L nitrate. Meanwhile, many freshwater species of fish can handle over 100 mg/L nitrate.
Similar to solids, as an operator of a RAS approaches zero discharge, nitrate levels will accumulate to higher levels at a faster rate. Nitrate is typically reduced by using water renewal as the primary method. All or a fraction of required water renewal amount is achieved while also removing excess solids from the RAS filters.
Aquaculture wastewater comprises of water and solids that are high in organic material, nitrogen and phosphorus. It is important to characterize wastewater for levels of suspended solids, total nitrogen, total phosphorus, pH, salinity (total dissolved solids) and ash, as well as for dissolved concentrations of ammonia, nitrite, nitrate and orthophosphate. If discharged directly to the environment it can cause great harm.
Another important factor is to understand the volume and schedule of when you need to discharge wastewater. These factors combined will provide important information, termed loading rates, that will help drive an aquaculture facility’s wastewater management plan.
The location of the aquaculture facility is critical for determining how to manage its wastewater. More specifically, what resources are available and what the local environmental regulations are.
If land area is not a constraint, a properly designed constructed wetland can be used to treat aquaculture wastewater. Wetlands use various natural processes to treat or “clean” aquaculture wastewater: quiescent pools allow suspended solids to settle out, wetland vegetation remove nitrogen and phosphorus from the water column, and organic matter can be assimilated or oxidized to carbon dioxide via naturally occurring microbes in the sediment.
If the site is located in a town or city, there may be an option to work with the local municipal wastewater treatment plant by discharging aquaculture effluent directly (or after some pretreatment) into the sewer system. Since aquaculture wastewater is different in composition compared to the typical wastewater it receives from the town or city, they will need to make an assessment to determine whether or not they can receive it. For example, if the effluent water is from a marine RAS it will contain a lot of salt and if the wastewater treatment plant is not large enough, it may not be able to receive the aquaculture wastewater as the salt will disrupt the freshwater biological processes and could be catastrophic for its overall operations.
If discharging aquaculture wastewater to a municipal wastewater treatment plan is not an option, then alternative engineering strategies can be implemented. One strategy is to use biological reactors to digest the solids and improve wastewater quality. Types of biological reactors that are implemented include, but are not limited to, sequencing batch reactors, continuously stirred tank bioreactors, packed bed bioreactors, and membrane bioreactors.
If designed and operated appropriately, the use of biological reactors can be a great strategy for managing and improving the quality of aquaculture wastewater prior to being further reused by the RAS facility, sent as pretreated wastewater to the sewer, or discharged to the environment as treated effluent water.
Aquaculture wastewater or spent-wasted solids from biological reactors should be dewatered prior to transportation to its final destination whether it is a landfill or used as a fertilizer (depending on local environmental regulations). Dewatering is the process of removing water content from the solids/sludge, resulting in material that is more consistent with cake.
Dewatering can be relatively low-tech. Concentrated wastewater or sludge is placed in a large, sealed bag with a tiny hole which allows the water to leave the bag but not the solids. These bags are often placed on earthen pads. More advanced technologies can also be used to dewater, such as specialized centrifuges or filter presses. Polymers may be added to the sludge to improve the efficiency of the process.
Each RAS operation is unique and producers should work with appropriate experts, engineers and local authorities to come up with a sustainable wastewater management plan that will lead to success while minimizing their impact on the environment.
David Kuhn is a civil and environmental engineer and an associate professor and extension specialist in the Department of Food Science and Technology at Virginia Tech in Blacksburg, Virginia.
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