Institute for Water Education, Wageningen University, 6708 PB Wageningen, THE NETHERLANDS.
The costs of mainstream wastewater treatment technologies, both in terms of construction as well as in terms of operation, keep many small to medium sized communities back from utilising reclaimed water for unrestricted reuse purposes. These treatment technologies are not only expensive, but also do not allow nutrients recovery. The main purpose of this research study was therefore the development of a sustainable treatment scheme to recycle nutrients and water. Use of an integrated UASB-duckweed ponds system for domestic wastewater treatment and recycling of nutrients and water in tilapia aquaculture was investigated. Monitoring of the integrated UASB-duckweed ponds showed that the overall efficiency of the system for organic matter removal (COD, BOD and TSS) was not significantly affected by temperature. Effluent quality was 49 mg COD/l, 14 mg BOD/l and 32 mg TSS/l in summer (24-34 °C), in comparison to 73 COD/, 25 mg BOD/l and 31 mg TSS/l in winter (13-20 °C). Nutrient removal was significantly reduced in winter, which shows that the nutrient removal processes are more temperature dependent than the BOD and COD removal mechanisms. Residual values of ammonia, TKN and total phosphorus were 0.4 mgN/l, 4.4 mgN/l and 1.1 mgP/l in summer, in comparison to 10.4 mgN/l, 12.2 mgN/l and 2.7 mgP/l in winter. The faecal coliform count in the fmal effluent was 4.0x103 ± 3.7x103 cfu/100ml and 4.7xl05± 5.5x105 cfu/100ml in summer and winter, respectively. A nitrogen mass balance in summer showed that the most important removal mechanism for nitrogen in the duckweed ponds was plant uptake (80.5%), followed by denitrification (14.8%) and sedimentation (4.7%). Ammonia volatilisation was negligible in all ponds. The nitrogen uptake ranged from 4 to 4.9 KgN/ha/d in summer and from 1.2 to 1.5 KgN/ha/d in winter. For phosphorus it ranged from 0.86 to 0.97 Kg P/ha/d in summer and from 0.27 to 0.32 Kg P/ha/d in winter. The duckweed production rate was 126-139 kg dry matter/ha/d in summer and significantly decreased in winter to 31-36 kg dry matter/ha/d. These results showed that the UASB-duckweed ponds system is technically appropriate for sewage treatment in small communities and rural areas and provides marketable by-products (duckweed biomass).
Nutritional value of duckweed biomass harvested from the duckweed ponds was evaluated for raising tilapia using semi-batch experiments. The duckweed (Lemna gibba) was evaluated as sole feed source for Nile tilapia at temperature range of 16-25 °C using fresh and dry duckweed at different feeding rates. Statistical analysis showed significantly (p < 0.01) higher specific growth rate (SGR) for tilapia fed with fresh duckweed than for the parallel treatments fed with dry duckweed. In case of fresh duckweed, the SGR of tilapia fed on 10% feeding rate had a significantly (p < 0.01) lower value than was observed for 25% and 50% feeding rate. There was no significant (p >0.05) difference between 25% and 50%. In case of dry duckweed no significant (p >0.05) differences were detected between the 25% and 50% feeding rate. Results of the nitrogen mass balance showed that the nitrogen content of dry sediment (solid waste) ranged between 4% and 10.5%. In case of optimal feeding rate (25% fresh duckweed) the value was 4.5%. This amount of nitrogen represented 8% of the dietary nitrogen input while nitrogen recovery by fish represented 26-28% of the total nitrogen input.
Use of both treated sewage and duckweed biomass from the UASB-duckweed pond system was evaluated in rearing Nile tilapia (Oreochromis niloticus) in ponds. Comparison of fresh duckweed with local fish feed ingredient (wheat bran) and commercial fishmeal based diet was investigated. Three different water sources; freshwater, treated effluent of the UASB-duckweed ponds system and settled sewage were investigated. The results of growth performance proved that, in case of freshwater ponds, the SGR of tilapia fed on fresh duckweed was significantly (p < 0.01) higher than the SGR of tilapia fed on wheat bran, but lower than the SGR obtained with fishmeal-based diet. In case of treated effluent fed ponds, there was no significant difference (p > 0.05) between the three feeds. This indicates the potential of duckweed to replace wheat bran as supplemental fish feed. For all three feeds the treated sewage fed ponds provided significantly better SGR than in the freshwater fed ponds. A negative net yield (-0.16 ton/ha/year) was obtained in a settled sewage-fed fishpond, due to high mortality in fish biomass (60% in the adult fish and 38% in the fry). This mortality was attributed to an average un-ionised ammonia concentration of 0.45 mg N/l during the autumn (mortality period).
Effects of chronic un-ionised ammonia nitrogen (UIA-N) concentrations on the growth performance of tilapia (Oreochromis niloticus) fed on fresh duckweed (Lemna gibba) were investigated. Statistical analysis of SGR showed no significant differences between the SGR of tilapia in the control (0.004 mg UIA-Nl-l) and the SGR of tilapia exposed to 0.068 mg UIA-N l-1 . The SGR of tilapia exposed to un-ionised ammonia nitrogen over 0.068 mg UIA-N l-1 (0.144, 0.262 and 0.434 mg UIA-N l-1) was significantly reduced. The maximum no observable effect concentration was 0.068 mg UIA-N l-1, while the lowest observable effect concentration was 0.144 mg UIA-N l-1. No fish mortality was detected in this trial, even at 0.43 mg UIA-N l-1 . Such a concentration caused serious mortality in the experiment with a settled sewage-fed fishpond (see above). The mortality in that pond may possibly be due to synergistic effects of other water quality parameters, like sewage bacteria count and low dissolved oxygen at dawn, to the toxicity of UIA-N. The results of microbial quality of tilapia reared in four faecal contaminated ponds support this. In spite of absence of any significant differences between the pond water microbial quality in treated sewage ponds and settled sewage fed pond, significantly higher contamination was observed in fish organs in the latter. This strongly supports the hypothesis of interrelated synergistic effects between microbial quality and some physicochemical parameters like ammonia, nitrite and/or low dissolved oxygen at dawn. The poor water quality in settled sewage-fed pond resulted in faecal coliform counts in fish organs of about one log10 higher than in the treated sewage fed ponds. Therefore pre-treatment of sewage to remove pathogens, organic matter and nitrogenous compounds is therefore recommended before use in aquaculture.
Apparent digestibility coefficients of duckweed (Lemna minor) were measured for Nile tilapia (Oreochromis niloticus). Tilapia was fed on four iso-nitrogenous treatment diets in addition to control diet. Effect of partial replacement of control diet components with 20% and 40% dry duckweed and 20% and 40% fresh duckweed was investigated. Specific growth rates of tilapia were l.51±0.07, 1.38±0.03, 1.31±0.06, 1.44±0.02 and 1.33±0.05, in control and treatments 1 to 4. All of the treatment diets provided good values for feed conversion ratio (FCR, 0.98-1.1) and protein efficiency ratio (PER, 2.49-2.78). All the treatment diets had high protein digestibility (78%-92%) and highenergy digestibility (78.1%-90.7%). lt is therefore possible to use duckweed as replacement of fishmeal and other plant ingredients in fishmeal-based diets at 20% and 40% inclusion. For 20%o duckweed inclusion there was no significant difference between dry and fresh duckweed for the apparent digestibility coefficients of dry matter, protein, fat, phosphorus and energy. However, in case of 40%o inclusion the fresh duckweed had significantly lower digestibility coefficient values for dry matter, protein, fat and phosphorus. The milling process might have had positive effects on the apparent digestibility of dry duckweed through destruction of cell walls and decreasing the particle size, consequently increasing the specific surface area for enzyme reactions. Body composition showed that tilapia fed on diets with duckweed has significantly higher phosphorus and protein content and a significantly lower lipid content. This might be attractive for the consumers. In conclusion treated domestic sewage and fresh duckweed (Lemna gibba), from a UASB-duckweed ponds could be used in semi-intensive tilapia pond culture. The duckweed could be used in intensive tilapia culture either as partial substitute of fishmeal or complete substitute of some plant ingredients.