Development of a sustainable wastewater treatment scheme to recycle sewage nutrients and water in
tilapia aquaculture was the main objective of this PhD research. Use of an Integrated UASB-duckweed
ponds system for domestic wastewater treatment linked to tilapia aquaculture was investigated. The
treatment system was efficiënt in organic matter removal during the entire year, while nitrogen,
phosphorus and faecal coliform removal were negatively affected by the decline in temperature in
winter. Most ofthe nitrogen removal was achieved by plant uptake (81%) while 14.5% and 4.5% of the
removal was due to denitrification and sedimentation, respectively. The treatment system provided
effluent quality and duckweed biomass suitable for reuse in tilapia aquaculture. The nutritional value.
fresh duckweed was significantly better than for wheat bran and similar to the commercial feed wher
used in combination with treated sewage from the duckweed ponds.
Using treated sewage enhanced the yield of tilapia and makes the combined feed of treated sewage
and duckweed similar in results to the fishmeal-based commercial feed. At un-ionIsed ammonia
concentrations above 0.1 mg UIA-N F^ the chronic ammonia toxicity to duckweed-fed tilapia
significantly reduced the growth. Other water quality parameters (pathogens, nitrite, low dissolved
oxygen) had synergistic effects on ammonia toxicity. Similarly, ammonia, nitrite and low dissolved
oxygen had synergistic effects on the faecal coliform count in tilapia tissues and organs. Therefore pf
treatment of sewage is preferable before use in aquaculture for maximum yield and quality assuranc*
The optimum feeding rate was 25 gram fresh duckweed per 100 gram of live fish weight at 16-25 °C.
The results on apparent duckweed digestibility showed a negative effect of the bulky volume of the ;
fresh duckweed at 40% inclusion while at 20% the digestibility coefficients were similar for fresh and
dry duckweed. The replacement of fishmeal, fish oil and plant Ingredients by 20% and 40% dry
duckweed or 20% fresh duckweed is nutritionally possible and economically attractive.
This research was carried out with financial support ofthe Dutch government within the framework o
the SAIL funded "Wasteval" project (LUW/MEA/971) and is the result of a cooperative effort of the
Water Pollution Control Department ofthe National Research Centre in Cairo, Wageningen University
and Research and the UNESCO-IHE Institute for Water Education.