People are disgusted at the thought of “untreated sewage” gushing into our rivers. I think they visualise floating turds and soiled toilet paper. But that is almost impossible if the sewage has passed through a Waste Water Treatment Plant (WWTP) like the one I visited in Ashford. We were given a special guided tour by two of the engineers employed by Southern Water. Grandchildren accompanied us and their account has already been published. Now it is time for a more technical analysis of what we saw and heard – and smelled!
The sewage from some 159,000 households flows down the pipelines to this WWTP, the second largest that Southern Water manages (the largest being near Portsmouth). This site takes not only piped effluent from the main drains but also has a facility that takes the contents from the lorries that collect by suction from the back garden tanks of some Kent villages, as they used to in Benenden during my childhood there.
The infrastructure here is robust and extensive, with flow pipes of large diameter (950mm), huge concrete circular tanks, swept continually by revolving arms, and looming bulbous gas containers. The construction of all this was begun in the 1960s, after it was realised that England could no longer depend on the Victorian system of sending untreated poo along large pipes to the coast and straight into the sea (sometimes flowing back onto the beaches where our families were attempting the sunbathe or sea bathe).
Waste water treatment stages
The first stage in the process of treating sewage is to remove the larger solid objects that get thrown down the toilet by the heedless public. There is a large filter that traps these. They include toys, underwear, nappies, keys and cell phones. Some WWTPs keep a fun display of these in the manager’s office. The children were rather keen to see this, but unfortunately Ashford does not have such a souvenir collection. There is a constant need for public education about such misuse of the flushing toilet (see this short film about the unflushables). Of course, some of this detritus is what mischievous toddlers have thrown into the toilet bowl, but a lot is just down to ignorance and laziness: for instance, flushing away wet wipes or menstrual products or pouring used cooking oil into the kitchen drain. These cause the dreaded “fatbergs” that cost the water companies so much effort and money to remove from the sewers.
After these are removed, there are sedimentation tanks that remove the soft biosolids, mostly mixed with tissue. This is then collected into heaps to be processed into compost. Some of it is sold back to farmers. But they are still adjusting to the market. Making pellets from it may make it more saleable. The fact that it is not chemically treated does not matter. As gardeners know, after a certain time compost, even from the most rotten and smelly material, becomes usable fertile soil by the activity of the bugs that feed inside it.
Gas (methane) is emitted in this sedimentation process which is then used to power generators for the electricity used on the site.
There may still be some particles in the sewer liquid as it emerges from these filters so some iron is added to help these drop to the bottom of the tanks. The liquid flows into huge round tanks, more than 16 metres in diameter and 3 metres deep, with a rotating arm that stirs the liquid to assist this process of removing the particles. However, the liquid is still somewhat brown when it flows along to the next stage, which is the huge settling tanks. These are full of rubble which is covered in slime. The liquid seeps through this, being purified by some specific microbes (which live in the slime). Gary, the engineer, was very emphatic that the site does not use chemicals to disinfect the water: it is all a bio-process.
It takes a day or so to get through this filtering. What emerges is clear water as can be seen when it overflows the rim of the settling tanks into the open concrete outlets that have been constructed for the outflow. There was some foam in the surge just before the main outflow which the manager said was because at this time of year the bugs shed their skins.
There are at least two monitoring stations along this route. Each of them was being checked by an inspector when we visited. There are details of the permissible limits of chemical composition of this liquid posted on the management office wall.
Finally, the clear water flows out to the Stour . There is some foam here on the surface too, but that is just because of the force of the current, the manager said. On the opposite bank there is a popular dog-walkers path, so this outflow is quite visible to the public. Sometimes people phone in about the foam. The staff check back at the monitoring records. But mostly it is just a false alarm because there has been so much publicity about “untreated” sewage that people are nervous about the foam.
If there is a filtering failure, Tony pointed out several times that there are procedures for sending back the flow to run through the filters again. However, within the system managed by Southern water (368 Waste water treatment plants), there are incidents of storm water overflow into rivers or out to sea (see answers to questions here). The government target is to stop all stormwater overflows by 2050, see DEFRA plan issued Sept 2022.
We walked back from the river along the path through the floodplain. He did not explain whether that is ever used for stormwater overflow. It is several acres covered in vegetation which would presumably soak up excess water.
We could see the looming gas tanks, but because we were with children we were not allowed to visit that section. The manager explained that most of the energy from the site is generated by the bio-gas that comes from the sewage processing. Only some of the offices are connected to the national grid.
I questioned Gary about the capacity of the site to take all the extra sewage that is added yearly from all the large housing developments around Ashford. Is there a need to build more tanks to cope with this increase? If so, shouldn’t the developers be contributing to the extra construction costs? Gary replied that there is enough capacity IF there wasn’t the need to allow for the stormwater increase, as they have to do with their calculations including climate change predictions for rainier months.
I gawped at the size of the empty round filter tanks, and speculated how much it would cost, and how much concrete would be needed, to construct even one more of those. It needs to be pointed out that hard engineering solutions to river and beach pollution are expensive not just for the water companies but for climate change too, as the construction is carbon-intensive – all that concrete and lorries bringing it to site!
This is why Southern Water are very keen on more natural ways of preventing excess storm water from entering the system at all. There are schemes (in Deal) to utilise more water butts. It is important that houseowners do not increase rainwater entering the sewage system by house extensions or conservatories with downpipes into the foul water underground sewers. Also, as more people tarmac their front gardens for car-parking this is so much less soak-away for rain. And everyone who uses a flushing toilet or a kitchen sink must be made aware of what SHOULD NOT be dropped down into the public sewer system. There needs to be more lessons about this in schools. There is excellent pictorial information on Water for Life sites. And there should be more information which goes with the sale of modern sanitary ware and kitchen ware but, in the rush to buy and sell, what is in the public interest gets ignored.
I do not think the onus should be only on the water companies to prevent sewage outflows: many more should be involved: builders, planners, farmers, gardeners, house-holders… just about all of us who use modern conveniences.
