Two-stage high-load digestion

The procedure

The optimization of operating conditions in sewage sludge digestion was the subject of intensive research work at Fraunhofer IGB. The result is the two-stage Schwarting-Uhde process (now Schwarting Biosystems GmbH), which was patented as early as 1979. The process, which has considerably improved efficiency, a short residence time and a high degree of degradation, is used for the anaerobic conversion of organically degradable substrates such as liquid manure, biowaste or sewage sludge. Due to the significantly increased biogas yield, it is becoming increasingly popular.

Operational data of the process compared to conventional digestion

Shorter dwell time

Even with high solids content, the sewage sludge can be converted with a residence time of only 5 to 7 days. Conventional digestion towers are operated with an average residence time of 20 to 30 days. Thus organic space loads of
8-10 kg oTR / m3 d instead of 1-2 kg oTR / m3 d reached.


Higher biogas yield

With high-load digestion, biogas production increases to up to 23 litres of biogas per inhabitant per day. In contrast, conventional digestion achieves an average of only 19.7 liters of biogas per inhabitant per day [Haberkern et al. 2008]. The gas can be used to supply energy to the plant or to dry the sewage sludge or can be released as a technically and commercially usable energy source.


Fewer digestate residues

In the course of increased biogas production, high-load digestion also reduces the content of organic ingredients - by 50-70 percent depending on the specific process combination. The organic content of the dry residue is now only 50 percent. The sludge can therefore be dewatered more effectively. This results in much smaller sludge quantities that can be disposed of at low cost.

Further improvement through microfiltration

Diagram of the two-stage high-load digestion with microfiltration.
Diagram of the two-stage high-load digestion with microfiltration.

The extension of the high-load digestion process by microfiltration with the rotating disk filter, an energy-optimized and low-maintenance filter with ceramic membranes developed at Fraunhofer IGB, leads to further considerable improvements: As a result of the concentration of the biomass, the solids residence time can be shortened and the turnover and the achievable biogas quantity can be increased additionally. Further advantages are improved dewatering of the residual sludge, lower sludge volumes and thus reduced costs for sludge disposal. The particle-free filtrate is also rich in ammonium and phosphorus, which can either be recovered by stripping or precipitation and used as fertilizer or which can be used directly as fertilizer water 

Energy efficiency even for small wastewater treatment plants

One-stage high-load digestion plant in Ilsfeld.
One-stage high-load digestion plant in Ilsfeld.

Using the example of a wastewater treatment plant for 28,000 inhabitants, Fraunhofer IGB has shown in a cost-benefit study that it is worthwhile for smaller wastewater treatment plants to switch to the more energy-efficient high-load process - even if they have to invest in a sludge digestion system. The annual disposal costs of 225,000 euros for the digested sludge could be reduced by up to 170,000 euros if the sludge were not degraded aerobically but in a high-load digestion process with microfiltration.

Around 60 percent of the organic matter is converted into biogas using the high-load process with microfiltration - the yield is thus around a third higher than in the conventional digestion process. The biogas produced can be used to operate the plant. In the case study example from the study, this reduces energy costs by at least 70,000 euros a year.

A further advantage is that high-load digestion produces smaller quantities of residual sludge that must be disposed of. This saves the operator another 100,000 euros. This is because, in addition to the high energy prices, it is above all the rising disposal costs that are having a negative impact. The recycling of residual sludge in agriculture is controversial and in Baden-Württemberg it is now often dispensed with. Sludge may also no longer be deposited in landfills. However, the alternative of burning the sludge is very expensive. Effective sludge reduction by digestion is therefore worthwhile, especially for smaller sewage treatment plants of up to 30,000 PE, which until now have often stabilized the sludge aerobically with a high energy input.