A good question. This was the topic of Chris Digman's PhD and he developed a predictive model that is still used by MWH. Suggest you ask him directly on: [email protected]

I have some questions for you:

1. how widespread is nutrient removal, especially biological nutrient removal, in UK secondary operations of wastewater treatment plants?

2. do you have any figures on Cd in post AD sewage sludge in UK - and what goes to land? Is it building up in UK soils and amybe overseas?

The US tends to define biological nutrient removal as nitrogen or phosphorus removal. The UK tends to define it only as phosphorus removal.

Nitrification/denitrification to moderate levels (50% of produced nitrate converted to nitrogen gas) is standard practice in the UK. Higher levels are relatively rare.

Biological phosphorus removal comes and goes. At one point only Thames championed this. At the moment I am told that they are moving away from this and shifting to chemical P removal. Meanwhile Severn Trent, who regarded bio-P as unreliable, are now looking at it more favourably. Generally bio-P is not seen much in the UK. It has the reputation of being unreliable with the UK staffing levels.

Chemical P removal is relatively wide spread. I don't have numbers, but there should be UKWIR reports that give you that detail. There is a joint research programme where the water companies are looking at chemical treatment methods allied with filtration as the EA has indicated that consents will be revised, with a wider spread of P targets and many of these being below the old thresholds of 1 - 2 mg/l. I'll see if  I can find any of the presentations.

Cd I'll get back to you!

Cadmium the reply that I have been given is that the sludge to land regulations set limits in the soil (3 mg/kg dry soil. US is 20, Sweden is 0.4), so that it cannot accumulate to levels that would be of concern. Cd in sludges is generally at a level that would not be of concern (1.3 mg/kg dry solids), but there may be local hotspots that are not picked up in the aggregated figures. The local hotspots (if any) have three choices - incineration followed by landfill; keep finding new land for land disposal as the Cd limits are reached; or find land that is naturally high in Cd. I know that (for different metals) the latter has been permitted in a few places in the UK, but I last had the details in the early 1990s and could not be sure that such agreements have been allowed to continue.

Jeremy

I'm trying to find out what % volumes of UK sewage sludge now go to AD? I've had some conflicting data, with one YWS response saying that there are only 5% of works with AD. Of course a lot of sludge is being tankered around.

YWS could be correct that only 5% of works have AD.

http://adbioresources.org/docs/July_2015_Market_Report.pdf claims that there only 159 AD plants in the UK treating sewage sludge. https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/69400/anaerobic-digestion-strat-action-plan.pdf has a higher number, as it records 146 producing energy (I assume exportable energy).

But this ignores the size of the facilities. https://www.ofwat.gov.uk/wp-content/uploads/2010/02/rpt_com_le_upstream.pdf reports that 6% of the sewage works treat 82% of the sewage load/flow.

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/69592/pb13811-waste-water-2012.pdf reports that 75% of UK sewage sludge is anaerobically digested, and gives you a tonnes/year value for sewage sludge disposed of through the agricultural route - much of which will have been anaerobically digested, dewatered and then stored for 6 months or more.

Thanks Jeremy - I need this info for the Commons Select Committee inquiry on food waste