I'm sure most of you have wondered about what flows from the Outfall at the top of Beat 3b at Harpers, especially when various odours waft downstream from there. Last week, with Mike Wheatley, we visited the source of that water and the odours, for a guided tour of the Horsmonden Water Treatment Works (WTW) at the invitation of Southern Water. We had an excellent tour, explaining all the stages and processes, and explaining a few misconceptions about what goes on there.
First of all, it is a Water Treatment Works, not a Sewage Works. It takes the contents of drains from the Horsmonden area (7,669 population) and treats that liquid to remove the solids, as sludge, and treat the water so that it meets the approved standards for disposal into the river. This is partly through some chemical additives (primarily to neutralise the phosphates) but mostly through aerobic digestion by bacteria, in chambers like this.
The treated water then goes through further refining to remove more solids until it is clean enough to be released.
A few points about the processing:
The released water is tested automatically every hour and is also subject to random testing without notice.
The site cannot be overwhelmed by heavy rain etc because its input is all pumped to the site, not directly fed from drains, so it is always controlled, and the site has up to 3x capacity of normal dry flow conditions.
Any chemical, perfume or other smell from the output is not an additive, but a function of whatever people put down their drains, e.g. perfumed detergent, which cannot be removed.
The tankers that can often be seen from the riverside do not discharge anything into the WTW. They are there (daily) to remove the processed sludge from a large silo just inside the fence, to take it to a Sewage Treatment Works elsewhere for processing. There are two very large empty chambers which can store any excess sludge in case the tankers can't remove their daily cargo for any reason.
If SW fail to meet the approved levels (e.g. <1mg/l of phosphates) they can be fined millions of pounds, so the processing is very carefully monitored and controlled.
I hope that clears up some of your worries about the outfall. No doubt it would be better if the river didn't have such an addition, but it has to go somewhere, and the WTW seems to do a pretty good job of making it as safe as possible. The fish certainly seem to like it, even the fussy ones like Grayling.