This presentation reviews the twist and turns of sludge technology and biosolids management from one who has presented all the conferences since 1995 except two. Hopefully there are some lessons learnt that can inform future investment and practice.

The early years of the conference were dominated by mild panic! With the enactment of the Urban Wast Water Treatment Directive in 1991 and the passing of the Oslo convention that banned dumping of sludge in the Sea in 1988 there was a rapid growth in the quantity of sludge to be treated from 1 million dry tonnes to 1.5 million dry tonnes per year. This combined with some immaturity in land application techniques led to a major capital spend on Incineration and thermal drying; at that time only 40% of sludge went to land. The immaturity was typified by some unsustainable land application practice such as raw sludge lagoons, raw sludge sub surface injection and liquid sludge surface spreading. Plus, the early days of cake dewatering were dominated by poor quality products. I know- I did some of this in the 80s as a biosolids manager at Thames Water. There were a lot of papers on thermal drying and destructive technologies in 1995 -2000.

The next big factor was the development of the Safe Sludge Matrix; I was involved in the early phases when involved in the Biogran operation out of the Avonmouth dryer 1996. This had the general effect of Water Companies focussing more on quality of sludge. In the early 2000s the era of cheap energy changed after OPEC imposed price controls on oil and gas. Dryers and Incinerators became problem children due to energy costs and complexity. Temporary measures became popular – this was the era of lime stabilisation as a quick fix to get raw sludge treated. I developed some lime stabilisation projects at that time. Composting was tried but never succeeded due to management complexity.

The big change in the early 2000s was the realisation that biogas was valuable and that digestion could be improved. The early push was to get as much sludge through digesters as possible by increasing dry solids % input and reducing retention time. This was the era of rapid growth of advanced digestion with some form of pretreatment – at this time both hydrolysis and disintegration became main conference topics. I was working with both hydrolysis and disintegration. In the end disintegration fell away because it was not cost effective. The remaining big noises by the end of the 2000s were enzymic and thermal hydrolysis (THP). Also, around this time the concept of sludge centres was becoming well established as Utilities focussed on upskilling and centralisation of digestion and dewatering.

In the 2000s THP pulled clear ahead. One of the unexpected consequences was that customers realised the advantage of a superior product that could be stored and spread safely without nuisance; as an ex biosolids manager I always saw this, but it was hard to justify the economics; it was the icing on the cake of advanced digestion but it has helped enormously with acceptance and resilience of biosolids projects.

The upshot of this is the growth of land application has grown from about 400,000 raw dry tonnes per year in 1992 to about 1.5 million tonnes today = nearly all of the sludge produced is recycled to land. Now PFAS and Microplastics – Panic again! Well, we have had panics before over metals, PCBs, plasticisers and gender benders. Usually, the problem has been solved by source control and biosolids continue to be used safely. This time it may be different – it’s best to see what the USEPA and the EA have to say. I’m very encourage by Ian Peppers work and hopefully the USEPA will be reasonable and not bow to pressure from alarmists.

However, the digestion sludge centre concept when combined with high solids dewatering is the best first step to a thermal destruction route anyway – so expect a lot of papers on pyrolysis. Also, there will be some more novel solutions popping up. My advice is steady as she goes and focus on fugitive methane emissions. One or two Water companies are trialling pyrolysis – watch them and be ready if necessary. As an ex biosolids manager I am proud of what the Water Industry has done; the products we make and service we offer are light years ahead of where we were 30 years ago.