Sewage treatment is done in different steps and the algae problem comes at around the last or the second last steps. Currently treatment plants are spending money to remove the bio particles etc which can be used as fertilizers. These cannot be part of the purified water they let out, thus it needs to be removed. Private companies, individuals or co operative societies have an opportunity here, they could collect this waste and/or extract algae from the treatment plants ponds.
The algae extracted could be directly used for oil production after processing. The Bio particles collected could be used as fertilizers in algae ponds. Thus making the algae oil a lot more Eco friendly than when chemical fertilizers are used. This can be effective waste management methods where land is available, land is required because algae ponds need more surface area than depth for maximised algae growth.

Another method would be to use :-

Thermal depolymerization, this uses hydrous pyrolysis to convert reduced complex organics to oil.

But the problem with this would be environmental and economic issues. The amount of heat used in this process is huge thus making it economically non viable and environmentally ineffective. So Biodiesel would be the better option.

Thus our own waste can power our vehicles.