A possible future use for waste water (sewage as well agricultural) and flood plain run-off is as nutrition in algae farms. Water like this do however contain a lot of bacteria that are harmful to lipid rich algae strains and the waste water would therefore need to be treated before it is used. The water would first have to be processed by bacteria in a process called anaerobic digestion If the water isn’t treated correctly, the algae growth can be stunted and large part of the algae population can die off.
On a related note, the end product from biogas facilities is very easy to process to make an ideal algae farming medium.
One of the main criticisms about algae farming is that it would be a waste of valuable fresh water. This problem is avoided if waste water or salt water is used for the farming, and methods using these types of water are therefore strongly preferred by most scientists. There is however a fear that pollutants and heavy metals in waste water might stunt algae growth and make waste water growing a non-viable alternative. The same can be true when using salt water although the concentration of heavy metals is usually lower in salt water compared to waste water. Some scientist believe agricultural-grade fertilizer is the best way forward but the same heavy metal problems can be found here as well. The solution might be to find and use algae strains that are more resilient towards heavy metals. There are several types of string algae used in open pond farming that exhibit a very high resilience towards heavy metals.