How do you filter out low quality content for a site that is open to publish content for all?
Constant moderation of content is the best possible solution here.
You need strong moderation stuff for this. Who will not only filter out bad content but replace them with good content. And the work is not just filtering out but also to correct them grammatically and by any other means necessary.
We have hired staff to do this for us. If you own a site that constantly generates content in its own then you need staff to regulate them too.