Possibilities and limits of wastewater-fed aquacultures

Wastewater-fed aquaculture offers means to treat wastewater with integrated material-flow recycling. Several goals are achieved simultaneously: production of valuable goods (food stuff, animal feeds, raw materials, ornamental plants and animals) on one side, and production of utilizable gray water (wastewater purification and hygienisation) on the other side. The main potential of wastewater-fed aquaculture and its major advantage over conventional wastewater treatment is the large diversity of marketable products and therefore broad possibilities of income generation. The combination of the two income generating options (wastewater treatment and biomass production) is a very interesting feature and in addition complies to several global political programs (like Agenda 21). Aquaculture is facing challenges. Optimal stocking depends on biogeographical conditions (which species grow where, under what circumstances), cultural acceptance (which products are suitable and marketable) and economical conditions. Among factors limiting the potential and performance of aquaculture are: limited growth rates of organisms, insufficient knowledge of the factors that regulate the aquatic community, the presence of toxic contaminants (heavy metals, hormones) and other undesired effects (colorations) in the wastewater. Appropriate technological tools (aeration, mixing, pumping, special basin forms) can be integrated in order to intensify certain ecological processes and increase the output of the aquaculture plant. At the University of Applied Sciences Wädenswil, Switzerland, wastewater-fed aquaculture is a research focus since 1993. This paper summarises some of the results and insights gained during the past seven years and gives a short overview of literature

The Otelfingen aquaculture project : recycling of nutrients from waste water in a temperate climate

A wastewater-fed, partly indoor aquaculture plant (36 basins, 360 m2 and 420 m3 in total) was designed in Otelfingen/Zurich, Switzerland. It was charged with the effluent from a methanization plant processing organic household waste and started operation in spring 1998. The aim of the successive arrangement of the different modules and steps was to efficiently recycle water-borne nutrients in the form of aquatic biomass products, such as floating (ornamental) macrophytes, fish, zoo- and phytoplankton, suitable for selling on the Swiss market. Besides treating the effluent (total organic carbon [TOC], total nitrogen, nitrate [NO3-N], ammonium [NH4-N], and total phosphorus concentrations being 670 g/m3, 255 g/m3, 150 g/m3, 95 g/m3, and 52 g/m3, respectively) according to Swiss law requirements, the research focused on the search for suitable aquatic organisms and their testing at different environmental conditions. During the 16-week experimental period, a total of 2,150 kg fresh weight (FW) ofbiomass (97% as floatingmacrophytes) was harvested. This way, 176 g/week nitrogen and 47 g/week phosphorus were eliminated by assimilation, corresponding to 25-35% of the system's inflow. Due to relatively high evapotranspiration rates (on average 35.4 mm/week) and for water reconditioning purpose in the fish stocking basins, fresh water was added. Nevertheless, the system's final effluent was very low (21% of total inflow plus rainfall) and was carrying only about 2% and 0.5% of the input loads of nitrogen and phosphorus, respectively. Hence, the elimination rate was significantly above the average performance of a conventional system normally applied in Middle Europe, although the concentration values of most parameters in the outflow were comparable. Macrophyte production (and thus nutrient assimilation) was close to theoretical maxima in basins with high nutrient levels. Both plankton and fish growth were, at their best, only moderately satisfying. The semi-continual planktonic microalgae culture, and therefore also zooplankton culture, could be improved if the light absorbing humic substances were removed in a pre-treatment. Under given conditions (i.e., temperate climate) fish would rather play an accompanying role in the ecological production process. A wastewater-fed aquaculture facility resembles an integrated production plant rather than a wastewater disposal site. In addition to that, it has potential to prove advantageous over the highly developed conventional wastewater treatment plants established in Middle Europe. Further research in this field is essential and also recommended, considering political programs like Agenda 21, which contemplate the need for sustainable strategies for handling resources

Effect of plant harvesting on nitrogen removal in constructed wetland systems

High nitrogen wastewater can be treated efficiently by a combined or hybrid constructed wetland system. The objective of this study was to find appropriate conditions for plant harvesting method. Four units of lab-scale combined constructed wetland module made of plastic sheet with 0.4x0.8x1.16 m were used. The removal efficiency was around 60-69% for nitrogen (TN). The reactor with half harvesting showed the highest nitrogen uptake in plant with 2.8 gN/g.dry wt. The plant growth in the module with cutting plants showed the highest growth rate, 0.8 cm/d in height and 1.4 plant/m2.d in density

Elimination of phenols, ammonia and cyanide in wash water from biomass gasification, and nitrogen recycling using planted trickling filters

Trickling filters were used to treat wash water from a wood gasifier. This wash water contained toxic substances such as ammonium, cyanide, phenols, and PAH. The goal was to develop a system that degraded toxic substances, and achieved full nitrification of ammonia. A 1 kW model wood gasifier plant delivered wash water for the experiments, which was standardized to a conductivity of 3 mS/cm by dilution. Toxicity was assessed by bacterial luminescence detection, germination test with cress (Lepidium sativum), and pot plants cultivated in a hydroponic setup irrigated continuously with the wastewater. Treatment experiments were done both in planted and unplanted trickling filters. Plant yield was similar to conventional hydroponic production systems. The trickling filters achieved complete detoxification of phenol, PAH and cyanide as well as full nitrification. The specific elimination rates were 100 g m-3 Leca d-1 for phenols and 90 g m-3 Leca d-1 for ammonium in planted systems. In unplanted trickling filters circulated for 63 h, phenol concentration decreased from 83.5 mg/l to 2.5 mg/l and cyanide concentration from 0.32 mg/l to 0.02 mg/l. PAH concentrations were reduced from 3,050 µg/l to 0.89 µg/l within 68 days. The assays demonstrated the feasibility of using the technique to construct a treatment system in a partially closed circulation for gasifier wash water. The principal advantage is to convert toxic effluents from biomass gasifiers into a non-toxic, nitrogen-rich fertilizer water, enabling subsequent use in plant production and thus income generation. However, the questions of long-term performance and possible accumulation of phenols and heavy metals in the produce still have to be studied