that's if all of them are goin hydro and all of them are dumping full res's I have got my res to run in pretty good order so I can let it drain down to 5 gallons before a res change...atleast half are growin in dirt and like I said that's spread out over the city....the vegetation in the areas around your house should have no problem soaking up the nutes...if you are that concerned however you could easily just let the used nute water sit for a while till the strength is appropriate for house plants like zandor said and just spread em around there...water being dumped down drains goes through treatment plants where they extract chemicals that are much worse for the environment than plant food trust me...aka bleach and various other chemicals that people use to clean tubs/toilets...

I still think the best thing is just giving house plants or yard the left over

PS the soil in the ground acts like a filter so if you dump a potent nute solution in the dirt in your backyard most of it would be filtered out after just a few feet of travel through the soil leaving a high concentration of nutes in the location you poured it...I have a hard time believing res changes are the cause of the increased algea I'd lean toward farms for that issue