Fresh Thinking on Sludge Management
Why have another conference about Sludge?
SludgeTech 2015 was instigated to bridge the gap between academia and industry. The industry felt the need to be more collaborative with academia, to promote good technical research and to prevent ‘sales pitch’ style papers. In our vision, SludgeTech is the event where quality science comes first and where researchers are encouraged to showcase their work in front of industry.
The conference was attended by Marie Roberts, Editor for the Water and Sewerage Journal Waterjournal_SludgeTech_article
What were the key learnings?
Anaerobic Digestion Science
- Imported sludge has a significant impact on AD performance and should be characterised to establish baseline plant performance;
- Variation in site and year explains a large part of the variation in E. Coli removal over two stage digesters. E. Coli input and primary digestion temperature were the only significant factors;
- Covered storage of biosolids was successful in achieving a stable stockpile temperature, which may enhance indicator die-off. Lab results showed an increased reduction in E. Coli concentration at storage temperatures above 25°C.
Thermal Hydrolysis Development
- Steam Explosion resulted in a decrease in particle size and an increase in the gas production (both greater and quicker). At higher pressures (up to 6 barg) the effect was greater;
- When sludge is treated centralised, THP is very benificial for VS removal, dewatering and the loading rate of the digesters. The energy production can increase when waste heat is used to make steam for the THP. N-removal in the reject water is important to reduce the N-load going to the head works;
- Intermediate THP (a THP unit between two digesters) can be applied on sewage sludge, no inhibitory effects were seen in the second stage The VS removal with iTHP is better compared to conventional THP.
Innovative Resource Recovery
- A WWTP can be transformed into a logistic centre, where sewage, organic household waste and manure are treated to recover resources. Manure can be treated with autothermal aerobic thermophilic treatment at similar costs as current practice;
- Post AD dosing of magnesium results in more efficient struvite removal compared to pre AD dosing. Removal of struvite results in a reduced nitrogen and phosphorus load to the headworks and less unwanted precipitation downstream;
- Gold, platinum and palladium originating from jewellery and catalytic converters are concentrated in sewage sludge incinerator ash at consistently high grades. Extraction is likely to be economically feasible, but development of extraction methods is crucial.
Dewatering and Sustainable Drying
- Cambi SolidStream can be used to increase dewatering (up to 40-43% DS) and gas production after digestion. The filtrate with a high suspended solids concentrations can be fed back to the digester instead of the head works;
- A reduction in shear viscosity in digested sewage sludge was concurrent with the increase of sCOD and process conditions seem to contribute to this. The best polymer dose region was indicated by an abrupt increase in network strength and low CST values;
- Sludge preheating (up to 60°C) is an effective strategy to enhance sludge dewatering, although the supernatant quality is deteriorated. Residual heat is crucial for the economic feasibility.
- Chemical contaminants that are present in sewage do not end up in large amounts in biosolis. All investigated contaminants were present at significantly lower concentrations than EU limits;
- Users of biosolids are willing to pay a higher price when more service is given;
- Sewage sludge digestate has a value, but this often does not cover the costs. Digestate upgrading might become essential, future facilities are therefore likely to be co-located with other businesses;
Advanced Energy Recovery
- Hydrothermal Carbonisation can be applied to convert PS into biocoal, with a 48% higher energy yield compared to competing technologies;
- Sludge is a physically and morphologically homogeneous material, although with high inert levels, making it a suitable feedstock for pyrolysis;
- Supercritical gasification is feasible alternative to incineration, since it can deal with wet biomass. At lab scale high conversion yields of up to 80% were achieved with sewage sludge;
- Sewage sludge can be an economically sustainable fuel in the long term, although more energy has to be extracted by advanced energy recovery, since AD leaves 45-60% of the energy in the sludge. To maximise returns, sludge capture and throughput should me maximised;
- Sewage sludge is an appropriate source for activated carbon, the best results (highest surface area) were achieved by with chemical activation by K2CO3.
What is the future of sludge?
The first day concluded with a panel discussion on the future of sludge. The speakers agreed that the biomass revolution is happening and sludge management should be part of this revolution. The focus should be on the extraction of high-value materials such as alginate and bioplastics. If this change is not made, the water industry will be outcompeted by other industries that are more focused on the revenues that can be gained from sludge. Right now, we should communicate and educate, we need to tell the story to convince the regulators, society, etc. of the need for change.