Most aquaculture systems in the EU are small, flow through operations discharging high volumes of dilute wastewater, and requiring specialized permits for their effluent streams. We found that pike perch, sturgeon, and aquaculture effluent is very nitrogen rich, generally matching the duckweed lagoon's removal efficiencies with minimal supplemental technology. While treatment lagoons would theoretically be cheaper than the bioreactor steps employed in recirculating systems, many aquaculture species grow best above 20℃, and the cost of reheating the water would be prohibitive. Growing rainbow trout and salmon in colder regions, or other species in warmer climates may help match fish tank and ambient temperature to enable flow through systems to make sense.
While flow-through systems currently discharge more solutes than other farms, duckweed treatment lagoons could be a cost-effective method of partially treating discharge water. While high flow rates may make high HRT difficult, inclusion of a coagulant, could substantially increase settling rate. Considering the dilute water is currently discharged to local rivers, an unlined duckweed treatment lagoon, might be legally feasible, and would reduce treatment costs by 33%.
For the modeling of a 500 ton per year pike-perch facility, we assume that drum filtering, aeration, ozonation or UV sterilization steps are carried out with or without a duckweed treatment lagoon to ensure that solids are removed, and that sanitized water is returned to the fish tanks, and therefore exclude these cost calculations. In the case study sheet we outline the CapEx and OpEx costs of biofiltration and denitrification with microbial reactors, and phosphorous removal via coagulant and clarifier. The Capital costs are largely devoted to the fluidized bed bioreactors for biofiltration and denitrification, totaling €285,000 which is assumed to depreciate over 10 years. The major OpEx expenses are the media for the biofilter, feeding of woodchips or other carbon sources to the denitrifying reactor, and FeCl3 or other coagulants for the before clarification. Overall, equipment and operations costs per year for N and P removal in a 500 ton per year pikeperch facility will run around €37,000 and €355,000 respectively, or €372,000 total per year, which we compared to the cost of duckweed treatment lagoons.
Using the tons per year of the closely related trout in the table above, we assumed that 20% of nutrients would be discharged at very low levels, and put the adjusted tons per year in the tons per year sizing sheet. Since a drum filter reduces the footprint by a factor of 5, we input values for post-drum filtered pike perch effluent. This showed that a 500 ton/year farm would require 5.3 ha of duckweed lagoons to capture all nitrogen and phosphorous. Adding in phosphorous coagulant would add €11,400 to the annual cost, but reduce the footprint to 3.35 ha, so we assume that Pike-Perch Farms employs the supplemental technology.
Since pike perch and trout require clear, fast flowing water, their aquaculture discharge meets the Flanders discharge limits for all nutrients, which is why flow through farms are possible where water costs and regulations permit them. This means that duckweed treatment lagoons do not need to be lined, which can reduce the installed cost per hectare from €276,000 to €171,000 and the annualized CapEx and Opex combined cost from €38,900/hectare to €23,200. With a total treatment cost of 3.3 hectares coming out to €130,300 we see that a treatment lagoon comes in as 33% of the cost of RAS treatment with a foil liner, and 19% of RAS cost without one. While this is impressive, it can’t compete with flow through systems. Another huge concern is the water temperature in both the fish tanks, and the duckweed lagoons. Since the ideal temperature to grow pikeperch is 25-30 ℃, and Flanders’ outdoor lagoon temperatures will generally range from 5-15 ℃ recirculating large volumes of water may suddenly become economically infeasible based on reheating costs, even with counter-current exchangers, and pressure changes influencing the temperature. While salmon and rainbow trout prefer colder temperatures of 8-14 and 13-18 ℃, and occasionally need cooling, most species grow faster in warmer waters. It’ll therefore be important to match fish species and duckweed cultivation in warmer climates and research and test technologies to reduce reheating costs to enable a recirculating system. Perhaps duckweed based treatment will fill a middle ground between flowthrough and RAS systems where water can either be cheaply treated before discharge or greatly reduce the cost of RAS for cold tolerant trout and salmon.
Based on the effluent of a sturgeon farm, which had a flow rate of 35 m3/hour of both pond overflow, and drum filtered effluent averaging 29 mg/L of TN, we found that 0.8 ha of duckweed treatment lagoons, with an HRT of 9 days would satisfy the TN, TP, and COD removal rates of the farm. The If the land is available for purchase, and an HRT above 3 or 5 days is possible, duckweed settling lagoons, look as if they can eliminate the need for a drum filter, if the goal is to recapture the nitrogen. The final cost of treatment for the sturgeon farm was €29,367 per year to meet the general discharge limits, in case permitting changes.