While using solids digestion ...

While using solids digestion at treatment plants, my advocacy is to use secondary effluent to feed algae to purify the water instead of chemicals. This takes longer but the algae do a better job so make full recycling of the water economically feasible and importantly you gain a high volume supply of biodiesel. This allows the WWTP to become the biofuel producer for the water district and even with price gouging the price at the pump will be 1/2 of fossil diesel so very popular with citizens to remove the captive-market profiteers from the loop. Growing algae from wastewater effluent is done for 1/3 of USA biodiesel producers, but none of this is coordinated as a process tied to normal treatment plants as one would expect in a unified design of sewage processing. So, to add to the above recommendations, adding the wastewater processing takes the use of photo-bioreactors to be able to grow 24x7 to keep up with volume with climate control for any climate or time of year, harvesting has a number of options as does processing the oil for use in vehicles.