RoKKa – Sewage sludge as a source of raw materials and climate protection at wastewater treatment plants

The path to the bioeconomy is a transformation process that changes established value chains and develops them into communicating value networks. In this course, Fraunhofer IGB is developing concepts for a complete transformation of wastewater treatment plants into wastewater biorefineries with the use of residual and waste materials.

Bioeconomy approach for local circular economy

This can be an important building block in terms of a local circular economy and a modern bioeconomy approach and is essential for closing material cycles. The focus here is on the recovery of nutrients and other valuable materials and the use of side streams such as CO₂ for the manufacture of downstream products. Products manufactured in this way are to be used iteratively in value-adding processes as starting materials in order to be able to realize a truly sustainable circular economy.

Solution approach: Process combination centered on recyclable materials

The project "Sewage sludge as a source of raw materials and climate protection at wastewater treatment plants" (RoKKa), which is funded by the state of Baden-Württemberg, pursues the vision of driving the development toward a wastewater treatment plant as a biorefinery by linking innovative processes in a value-centered, climate-friendly and participatory manner. By integrating the infrastructures at the wastewater treatment plants, which are already working efficiently for environmental protection, it will be possible to transfer the approach of a wastewater treatment plant as a biorefinery and increase its consistency.

High-load digestion enables valuable material production from sewage sludge and CO₂ utilization

RoKKa uses a total of six pilot plants at the Erbach (Danube) wastewater treatment plant to demonstrate the production of valuable substances from the partial stream of sewage sludge treated in a high-load digestion system. Nitrogen and phosphorus recovery is coupled with the production of microalgae. Carbon capture and utilization (CCU) as a basic chemical will be piloted with the CO₂ in the biogas stream. As a result of RoKKa, environmental protection goals of wastewater treatment plants can be considered multidimensionally in the future (water protection, bioeconomy, climate protection).

Outlook

The Ulm-Steinhäule wastewater treatment plant (440,000 population equivalents), which is only 20 kilometers away from Erbach, also plans to treat the sewage sludge in a high-load digestion system in the future. When the high-load digestion system is commissioned, a large proportion of the ammonium contained in the sludge will be dissolved back and fed back to the wastewater treatment plant via the sludge water. The AmmoRe nitrogen recovery process piloted in Erbach could be used here to reduce the nitrogen load on the one hand and to recover the nitrogen as a valuable substance on the other. In this respect, the participation of the Steinhäule sewage treatment plant in the RoKKa project is a good driver for implementing the piloted processes in a technical scale.

Sewage treatment plants clean our wastewater – in Germany, over 9 billion cubic meters per year. They not only remove organic impurities, but also large amounts of nutrients such as nitrogen and phosphorus. However, with conventional wastewater treatment, the important plant nutrients are lost: nitrogen compounds are converted into molecular nitrogen, which escapes into the atmosphere as a gas, using a lot of energy. Phosphorus is usually precipitated in the form of water-insoluble iron or aluminum phosphates, which are not available to plants, and disposed of with the sewage sludge – even though natural phosphate deposits for the production of fertilizers are becoming increasingly scarce.

After three years of research, development and operation, the project “RoKKa – Raw Material Source Sewage Sludge and Climate Protection at Sewage Treatment Plants” shows that sewage treatment plants not only treat wastewater, but can also contribute to a climate-friendly circular economy. At the Erbach (Danube) sewage treatment plant, ten project partners have piloted and tested forward-looking processes for recovering raw materials from wastewater over several months. A total of seven innovative demonstration plants were operated for this purpose, each of which can also be integrated as a stand-alone module into existing sewage treatment plants.

The results are also relevant in view of the entry into force of the revised Urban Waste Water Treatment Directive on January 1, 2025. According to this, stricter limit values for phosphorus and nitrogen elimination will apply in the EU in the future in order to further reduce nutrient discharges into water bodies.

Sewage sludge as a source of raw materials

RoKKa makes use of the established process of sewage sludge digestion, in which organic solids in the wastewater are fermented to produce biogas as a renewable energy source. Since 2016, a high-load digestion plant at the Erbach sewage treatment plant has been converting the sludge into biogas faster and more efficiently than conventional digestion plants. After digestion, the sludge is dewatered using a chamber filter press to reduce its volume. The filtrate from the dewatering process contains high concentrations of the plant nutrients phosphorus and nitrogen.

Increased energy consumption and nitrous oxide emissions due to nitrogen back-loading

Usually, the nutrient-rich filtrate from the sludge dewatering is fed back into the aeration tanks of the wastewater treatment plant; this back-loading accounts for 20 to 30 percent of the nitrogen inflow load of a wastewater treatment plant. Accordingly, it increases the energy consumption for the aeration of the biological treatment stages. In the aeration tanks, microorganisms consume oxygen as they convert not only organic substances into carbon dioxide (CO₂) and biomass, but also nitrogen compounds. The risk of back-loading is that the concentration of ammonium or nitrate in the effluent from the treatment plant increases, which in turn increases environmental pollution.

In addition, biological nitrogen removal leads to emissions of nitrous oxide (N₂O), which has an impact on the climate that is around 265 times stronger than that of CO₂. Using large-scale measurements, the University of Kassel has now been able to demonstrate in RoKKa that the recovery of nitrogen from sludge water and the resulting reduction in nitrogen back-loading into the main stream of the wastewater treatment plant can achieve a reduction in nitrous oxide emissions during biological nitrogen elimination.

The sewage treatment plant as a biorefinery

Since the higher the concentration of a substance, the better it can be recovered, RoKKa starts with the nutrient-rich sludge water. Instead of being returned to the aeration tank, the filtrate passes through various modules that can be used to turn sewage treatment plants into biorefineries.

Electrochemical phosphorus recovery with ePhos^®

The first module is the ePhos^® plant, a process module developed at Fraunhofer IGB for the recovery of phosphorus and nitrogen. With ePhos^®, phosphorus is electrochemically precipitated as magnesium ammonium phosphate, also known as struvit. The magnesium required for this is added in an electrolytic cell via a magnesium sacrificial anode, which is gradually consumed in the ongoing process. The product struvite can be used as a regionally produced long-term phosphorus fertilizer in agriculture.

The pilot plant installed at the collective sewage treatment plant in Erbach was designed to treat approx. 600 L/h. This corresponds to half of the full flow of filtrate water that occurs at the treatment plant. For the first time, a new process was used to separate the crystallized struvite, in which the precipitated phosphate salts were scraped off a belt filter. However, due to the low phosphate concentrations in the influent, the precipitation efficiencies in RoKKa were lower than in previous pilot tests, where 80 to 90 percent recovery was achieved.

The experience gained from the RoKKa project shows that the prerequisite for the efficient use of the ePhos^® process is the highest possible concentrations of dissolved phosphate in the sludge liquor. This is ensured by the operation of a biological phosphorus elimination (Bio-P) at the wastewater treatment plant.

Nitrogen recovery to ammonium sulphate fertilizer

In contrast to phosphorus, the filtrate water from the sludge dewatering in Erbach contained high concentrations of nitrogen. The approach investigated by Fraunhofer IGB in RoKKa involves recovering nitrogen as a fertilizer for use in agriculture by a process of chemical transmembrane absorption (TransMembrane ChemiSorption, TMCS) [1].

The recovery process works on the principle of gas absorption with membrane contactors and is highly selective for nitrogen. For this, it is necessary that nitrogen in the water is converted to gaseous ammonia (NH₃). The higher the pH and temperature of the wastewater, the higher the proportion of gaseous ammonia. A hydrophobic membrane in the membrane contactor retains the liquid flow but allows gaseous ammonia to diffuse through its pores to the other side of the membrane. Here, ammonia is absorbed by sulfuric acid, producing an ammonium sulfate solution.

Ammonium sulfate can be used directly as a regional fertilizer. In the RoKKa pilot plant, the ammonium concentrations achieved in the product solution were initially still relatively low. However, it was shown that the ammonium sulfate solution can be further concentrated to obtain an economically viable product.

Electrochemical formate synthesis from CO₂

The digestion tank of a sewage treatment plant produces a gas mixture consisting of approximately 65 percent methane (CH₄), which is rich in energy, and about 35 percent carbon dioxide (CO₂). Using a new process from Deukum GmbH, CO₂ was separated using an amino acid solution and recovered using an electrodialysis device. What remains is highly pure biomethane that can be fed directly into the natural gas grid.

The carbon dioxide (CO₂) separated from the digester gas is a potential resource for carbon-based platform chemicals. One way to convert CO₂ into a valuable material using renewable electrical energy is the electrocatalytic conversion to formate, the salt of formic acid.

In RoKKa, this process, which had previously been developed on a laboratory and pilot plant scale and with technical CO₂, was carried out for the first time with CO₂ obtained directly from the digester gas, and the target product formate was successfully produced in an aqueous solution. At around 45 g/L, the formate concentration was comparable to that obtained in previous pilot plant trials with CO₂ from the Fraunhofer IGB gas supply.

During the electrocatalytic CO₂ conversion, oxygen (O₂) is produced at the anode, similar to the case of electrolysis for hydrogen production. This oxygen often remains unused, but it can be used at the wastewater treatment plant to aerate the aeration tanks.

Microalgae cultivation: beta-glucan for plant strengthening

Microalgae are multitalented: through photosynthesis, they convert nutrient-rich process streams from the wastewater treatment plant and CO₂ from the digester gas into biomass and valuable storage materials. In this way, the CO₂ cycle was closed in RoKKa in addition to the nutrient cycle.

The project demonstrated microalgae cultivation in a novel flat-panel airlift photobioreactor system of Fraunhofer IGB with a volume of 125 liters. To do this, a process developed on a 6-liter laboratory scale was transferred to the new photobioreactor system with LED lighting. The photobioreactor system is equipped with a comprehensive sensor system for process control, which allows for partial automation. In addition to the filtrate water, which is rich in ammoniacal nitrogen, magnesium ammonium phosphate produced in the ePhos^® module was added to supply the microalgae with nutrients. This was done to compensate for the low concentration of phosphate in the filtrate water and to achieve an optimal nitrogen to phosphorus ratio.

The microalgae strain used, Phaeodactylum tricornutum, produces plant-stimulating polysaccharides, known as beta-glucans, under defined process conditions. These can help plants to defend themselves against fungal infections such as downy mildew and, in the future, will partially replace chemical pesticides, for example in viticulture. Alternatively, the harvested algal biomass can be used to improve soil quality.

The carbon from approximately two kilograms of CO₂ is bound per kilogram of microalgal biomass produced. The electricity demand for microalgal production is the biggest cost driver of the process. In the RoKKa project, an energy requirement of less than 150 kWh/kg of produced biomass was achieved for the two-stage process investigated. In order to further increase the economic efficiency of the process, the aim is to achieve an energy requirement of less than 100 kWh/kg for the two-stage process used by further developing the reactor and lighting concept and adapting the process control.

RoKKa: successful update for the wastewater treatment plant

RoKKa impressively demonstrates how existing wastewater treatment plants can be modernized and made more sustainable in order to improve their carbon footprint and recover valuable raw materials. New approaches such as nutrient recycling not only reduce the use of fossil raw materials, but also energy consumption. At the same time, the implementation of nitrogen recovery processes avoids the climate-damaging nitrous oxide emissions caused by the back-contamination of ammonium in the aeration tank. Expanded into biorefineries, wastewater treatment plants make a valuable contribution to raw material security and climate protection, thus contributing to resilience and to achieving national climate and sustainability goals.

The aim now is to implement the project results on an industrial scale. For this reason, the Ulm-Steinhäule wastewater treatment plant was involved in the project from the outset. With a design capacity of 440,000 population equivalents, it is ideally suited for transfer to a larger scale. The construction of a high-load digestion plant is currently being planned. As a direct result of the RoKKa project, nitrogen recovery is also being considered in order to minimize the negative impact on the sewage treatment plant. Meanwhile, the individual process modules such as ultrafiltration, ePhos^® and nitrogen recovery are available to interested sewage treatment plants for testing with real wastewater at Fraunhofer IGB or on site.