Sludge management on sewage treatment plants

Sludge digestion – alternative to sludge stabilization

Wastewater treatment plants remove organic matter from wastewater. If the accumulating sludge decays, biogas is generated as a by-product. However, only 10 percent of the more than 9,000 sewage plants in Germany have a digestion tank.

Smaller operations in particular baulk at the costs involved in building a new digestion tank. Instead, they enrich the sewage sludge with oxygen in the existing aerated activation basin, and stabilize it. Activating basins, however, require a lot of electricity and make the sewage treatment plants the largest municipal electricity consumers. At the same time, an enormous potential of energy is lost, since no biogas is produced during aerobic sludge stabilization. The digestion tanks of larger sewage plants are often out of date. They could considerably improve energy efficiency and cost-effectiveness by using the latest innovative technology.

Limited disposal possibilities

The disposal routes for sewage sludge from municipal wastewater treatment have already been restricted by the legislation. Landfilling is no longer possible. According to the amendment of the Sewage Sludge Ordinance in 2017, large sewage treatment plants (> 100,000 or 50,000 PE) may only spread sewage sludge as fertilizer until 2029 or 2032.

The incineration of sewage sludge will continue to gain importance, prices for disposal will increase. However, the alternative of incinerating the sludge is not sustainable because wet sludge does not make a positive contribution to renewable energy production. Aerobic sludge stabilization is expensive due to the high energy demand, often insufficient and not an adequate alternative for sewage plants > 10,000 PE.

High-load digestion of sewage sludge

The Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB has developed an energy-effcient high-load process for the digestion of sewage sludge. It was put into operation for the first time in 1994 at the sewage treatment plant in Leonberg. In the meantime, this process is being successfully applied by several other municipal sewage treatment plants. The outcome: The high-load digestion converts the sludge into biogas in a considerably smaller space and more cost-effectively than the conventional digestion towers.

Optimizing operating conditions for sewage sludge digestion has been the focus of intensive research at Fraunhofer IGB, and high-load digestion has been implemented at wastewater treatment plants using various process configurations. In addition to a two-stage process, single-stage operation is also possible. Even a high-load stage integrated upstream of conventional digestion pays for itself through higher biogas yields.

Intelligent utilization of sewage sludge as an energy carrier

The high-load digestion process developed at Fraunhofer IGB makes sewage sludge digestion a process that can, as a result of the effcient conversion of the sewage sludge contents into biogas, contribute substantially to the cost-effectiveness and energy effciency of sewage treatment plants. The process is therefore also suited for smaller treatment plants (10,000 PE) that so far stabilize the sludge aerobically with a high power consumption.

Sludge digestion – an energy-efficient alternative to sludge stabilization

The sewage sludge is stabilized with net energy production by means of high-load digestion, can be dewatered to a higher TS level and the residual sludge disposed of by incineration at the lowest possible cost. The regenerative energy carrier biogas is derived as a product. The thermal energy requirements of the sewage treatment plant can be covered by the biogas obtained and further expenses can be saved by means of combined heat and power generation. The high-load digestion process therefore also represents an economically intelligent alternative and considerably improves the energy effciency of municipal sewage plants.

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

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 

Using a wastewater treatment plant serving 28,000 residents as an example, Fraunhofer IGB demonstrated in a cost-benefit study that it is worthwhile even for smaller treatment plants to switch to the more energy-efficient high-load process – even if this requires investing in sludge digestion. The annual disposal costs of approximately 200,000 euros for the digested sludge could be reduced by up to 50,000 euros if the sludge were treated not aerobically but in a high-load digestion process.

About 60 percent of the organic matter is converted into biogas using the high-load process – making the yield about one-third higher than with the conventional digestion process. The biogas produced can be used to power the plant via combined heat and power generation in a CHP unit. In the case study, energy costs are reduced by an additional 50,000 euros annually due to savings on oxygen injection and on-site power generation.

Design data at small scale 

High-load digestion systems are individually dimensioned and designed with regard to their integration into the overall sludge treatment process of a wastewater treatment plant. For the successful realization of a high-load digestion, we therefore usually investigate the fermentability of the raw sludge in high-load operation beforehand on a pilot plant scale.

The experiments are carried out in a small-scale plant with automated 50-liter reactors at Fraunhofer IGB. Based on the characteristic data obtained here, we design a larger-scale plant and scale up the design to an industrial scale.

Pilot plant ensures optimum know-how transfer

In addition, we can implement the high-load digestion process on a pilot scale at the wastewater treatment plant. In this case, the tests are carried out in a pilot plant consisting of a temperature-controlled biogas recirculated reactor made of stainless steel with a useful volume of approx. 2 m³. This offers the opportunity to test the operation of a high-load digester on site and to smoothly transfer the process know-how to the operator to ensure successful implementation.

The circular economy is considered a key strategy for conserving resources and achieving climate targets. The constituents in wastewater from a sewage treatment plant can also be used as materials – if it is processed appropriately. The focus here is on nutrient recovery and the use of CO₂ to manufacture downstream products. These products are to be used as feedstocks in value-adding processes in order to realize a local and sustainable recycling economy.

Our high-load digestion process lays the foundation for the utilization of residual and waste materials, as it not only converts the sludge produced at a wastewater treatment plant into biogas as a regenerative source of carbon and energy, but also provides a nutrient-rich "sludge water" with the dewatering of the digested sludge and carbon-rich digestate with the concentrate as further usable material streams.

The sludge water is rich in valuable plant nutrients, above all phosphorus and nitrogen. Fraunhofer IGB has developed various concepts for recovering the nutrients from this water produced during sludge dewatering and processing it as fertilizer. Alternatively, the sludge water can be used as a growth medium for cultivating photosynthetic microalgae that grow with CO₂ and synthesize plant-stimulating polysaccharides, for example.

This approach has been pursued since 2021 in the project "RoKKa – Sewage sludge as a source of raw materials and climate protection at wastewater treatment plants", which is funded by the EU and the state of Baden-Württemberg.