Why don't they just? The answer is usually the same: it would cost more. Drilling operations regularly flare (burn) natural gas, because it would cost too much to transport it.

I misread your question, but I'll write my answer anyway:

Wastewater treatment plants do produce fertilizer. There is some concern about using on plants destined to be human food, due to the bacteria present in human waste, so it is often used on other crops

I am guessing because there's a lot more to growing and processing algae. What the article is talking about is just moving some of the spent grain to the water treatment plant.

Local farmers already partner with breweries to use some of the spent grain as cattle feed adjunct. I don't know whether spent grain is viable as fertilizer.

Another reason might be that most breweries are likely hesitant to introduce another microorganism in vast quantities into the brewery--especially if that microorganism feeds on the same stuff as yeast does--to avoid contamination of their main product: beer.

I think the cost of operating and running an algae farm for biofuel production is too expensive for the breweries. Another reason is the size of reactors or ponds for growing algae for biofuel is pretty massive. Most companies that grow algae for biofuel have multiple ponds the size of football fields and sometimes the yield is still pretty low.

I think working with algae farms would be the best way. They can provide the nutrients to the algae farmers. Which would be an easier additional revenue stream.

I think a better solution to cut the transportation would be to build a algae farm using closed systems next to the breweries or close to them

Spent grain is usually used as feed or for composting as it is.