A Case Study: Diffuse Pollution From Digestate Spreading

This map shows the location of the AD plant in relation to the Wye and Usk SAC rivers and its digestate spreading areas.

The Wye tributary that drains the spreading areas is the Afon Llynfi. Its phosphorus levels are 3 times the target set by NRW.

Could this be due to the sewage treatment works (black dot on the above map)? Of the two streams above the works, the Llynfi Dulas was also recorded at 3 times its P target, while the Trwffyd was 4 times its target. This would suggest another cause.

Meanwhile, over in the Usk catchment, NRW recorded a phosphorus reading in the Rhiangholl tributary in 2017 that was over 100 times its target. The sample also contained high BOD and ammonia values, which in NRW’s own words, suggested “a significant pollution event.” Even so, this appears not to have triggered a subsequent investigation.


A large anaerobic digester plant in mid-Wales processes manure from nearby Intensive Poultry Units (IPUs), amongst other materials.

It spreads its digestate on land locally, within the catchment of two Special Area of Conservation (SAC)-designated rivers that are failing their phosphate targets – the Wye and the Usk.

Recently, Powys County Council has granted planning permission for another IPU to be built locally with the extra manure generated being sent to this plant.

The local planners’ assumption is that because the facility is meant to be regulated properly by Natural Resources Wales (NRW), the spreading of its digestate will have no adverse effect on the SAC-designated rivers.

The reality….

The plant operator’s own records tell us the areas where its digestate was being spread in the Wye and Usk catchments from August 2018 to July 2021. After that date, no more information was displayed on their website.

Meanwhile, NRW has been monitoring some of the nearby streams.

The regulator has recorded extremely high phosphorus levels in streams that drain the spreading areas. These flow into the two SAC rivers failing their phosphorus targets.

Could there be another reason for the high phosphorus levels?

A sewage treatment works is located on one of the affected tributaries, which is a potential cause.

However, two smaller streams that enter this tributary upstream of the works also have very high phosphorus levels. Both drain digestate spreading ground.

Another stream within the spreading area flows into the Usk, the worst performing Welsh SAC river for phosphorus. It passes through a small village in its lower reaches before it joins the failing SAC. Otherwise, its catchment is very sparsely populated.

Despite this, Natural Resources Wales recorded in this stream the highest phosphorus levels in the whole of the Usk catchment.

Is digestate spreading the cause?

This case study does not give us definitive proof that digestate is causing high phosphorus levels in these streams.

However, it more than suggests a link and that digestate spreading is contributing to SAC failure.

What also concerns us is that no proper investigation has been undertaken by NRW, despite their own monitoring revealing these alarming results.

Another question also needs to be asked: what other, potentially harmful chemicals are coming from digestate spreading? High levels of ammonia, for example, can be fatal for fish.

It is reassuring that Welsh Government have “called in” the latest two planning applications for poultry units in Powys. Also, that Herefordshire planners no longer appear to believe sending manure to AD plants gives them certainty that no further damage will result from a development.

Existing ADs will continue to spread digestate of course, the effects of which NRW must start looking into.