Schriftenreihe Siedlungswasserwirtschaft Bochum 80 Anaerobic biochar filtration of municipal raw sewage for wastewater reuse Korbinian Kaetzl Herausgeber: Gesellschaft zur Förderung des Lehrstuhls für Siedlungswasserwirtschaft und Umwelttechnik an der Ruhr-Universität Bochum e.V. Bochum 2019 Von der Fakultät für Bau- und Umweltingenieurwissenschaften der Ruhr-Universität Bochum zur Erlangung des akademischen Grades eines Doktors der Ingenieurwissenschaften (Dr.-Ing.) genehmigten Dissertation vorgelegt von Korbinian Kaetzl Referent Prof. Dr.-Ing. habil. Marc Wichern Koreferent Prof. Dr. rer. nat. Harald Horn Tag der mündlichen Prüfung: 3. Mai 2019 Alle Rechte vorbehalten Wiedergabe nur mit Genehmigung der Gesellschaft zur Förderung des Lehrstuhls für Siedlungswasserwirtschaft und Umwelttechnik an der Ruhr-Universität Bochum e.V. Abstract In many fast developing economies, the agricultural sector accounts for up to 90 % of the freshwater withdrawals. Approximately 200 million hectares of the irrigated farm areas are located in low-income countries, which corresponds to roughly 75 % of the global irrigated areas. At the same time only 10 % of the generated wastewater in these countries undergoes an appropriate treatment. It can be estimated that up to 30 million hectares of the irrigated land rely on wastewater as irrigation water source. This is particularly the case for urban and peri-urban agricultural used areas of arid and semi-arid regions, where irrigation water during the dry season often consists of untreated wastewater or very heav- ily polluted water. The release of untreated or partly treated wastewater into the environment causes serious impairments with enteric microorganisms, as such as Escherichia coli. The situation is aggravated by high population growth and rapid urbanization, which increase the pressure on dwindling water resources, especially in Sub-Saharan Africa, where water demand is expected to increase by 300% between 2005 and 2030. Hence, there is an urgent need to develop a simple and efficient wastewater treat- ment system with locally available resources. A widespread implementation in low income countries will contribute to a substantial reduction of the associated health risks for farmers and consumers. Slow sand filtration (SSF) is one of the oldest and most suitable available low-cost treatment method for water purifi- cation. Its suitability as secondary clarifier effluent treatment to reach an appro- priate quality for safety irrigation water with a relatively high hydraulic loading rate (HLR) of more than 0.05 m·h-1 was confirmed by several studies. In this conjunction, biochar produced from agricultural residues was successfully tested as an additive in wastewater treatment systems. However, there is still a sig- nificant knowledge gap regarding the suitability of SSF as anaerobic treatment of raw wastewater and regarding the efficiency of biochar as filter material. In the present work, a set of experiments were conducted to investigate the appli- cability of anaerobic SSF or anaerobic biofiltration (AnBF) for raw wastewater treatment with respect to the reduction of fecal indicator bacteria (FIB) in wa- ter that is intended to be reused in agricultural production. Furthermore, the potential of biochar and its unpyrolyzed feedstock as locally available filter ma- terials were evaluated and compared with sand and gravel as common reference materials. For AnBF experiments, glass columns (d = 5 cm) with different filter materials were constructed and operated with municipal raw wastewater and a HLR of 0.05 m·h-1 at a constant room temperature of 22±1 °C. In a first set up, the suitability of AnBF for the treatment of pre-settled wastewater with a high concentration of suspended solids was investigated. Therefore, biochar that was produced from a grass (Miscanthus sp., Poaceae; effective diameter (d10): 0.86 mm) was compared with quartz sand (d10: 0.58 mm) as reference material. The -2 -1 mean organic surface load was 366±125 gCOD·m ·d , which resulted in a mean -3 -1 organic loading rate of 509±173 gCOD·m ·d . For the constructed biochar filters removal rates for E. coli were in the range of 1 to 2 log10 units and were thus higher and with more consistent results than for sand filters. The sampling of the biochar filter material at different filter depths indicated an additional retention of E. coli (around 0.5 log10-units) in deeper zones. Using biochar as filter mate- rial, also significantly higher elimination rates of the chemical oxygen demand (COD) and total organic carbon (TOC) could be achieved in comparison to sand. However, the elimination of FIB by both filter materials was insufficient and the high concentration of suspended solids in the influent water led to a rapid for- mation of a dirt layer (schmutzdecke) on the filter surface. Due to the relatively coarse filter material, filter blockages also occurred in deeper filter zones, where- fore large parts of the filter bed had to be replaced after 70 days and the test had to be stopped prematurely. As a result of these outcomes, an appropriate anaero- bic pre-filter (AF) had to be developed prior to AnBFs to improve removal rates of suspended solids and COD. For the construction of the AF, biochar and woodchips, both locally available and inexpensive materials in developing countries, as well as gravel as reference ma- terial was used and compared regarding removal efficiencies of COD, TOC, FIB -3 -1 and turbidity. Filters were operated with a mean OLR of 253±81 gCOD·m ·d -2 -1 and a mean organic surface load of 455±147 gCOD·m ·d . It was found that COD (up to 90 %), TOC (up to 80 %), FIB (up to 1.7 log10-units) and turbidity (ef- fluent turbidity below 35 NTU) could be significantly reduced. Hereby the per- formance of biochar filters was significantly more effective over the entire experi- ment compared to woodchip and gravel filters, indicating the superior properties of biochar as filter material for wastewater treatment. The findings of this exper- iment demonstrate the high potential of biochar AF for wastewater treatment in low income countries to reduce water pollution and to improve significantly the quality of water that is intended to be reused for crop irrigation. Hence, the re- sulted biochar filter effluent was expected to be suitable for further purification by AnBFs. Taking advantage of the encouraging results of both experiments, a two stage anaerobic wastewater filtration system with an AF followed by downstream AnBFs was developed. The AF was operated continuously with a mean OLR of 194±74 -3 -1 gCOD·m ·d for two years. The effluent of the AF was used as influent for the AnBFs. Also here, different locally available materials, namely rice husks (d10: 1.29 mm) and its pyrolysed equivalent rice husk biochar (d10: 0.66 mm), were used as filtration media and their efficiency compared with sand (d10: 0.34 mm). -3 -1 AnBFs were operated with a mean OLR of 63±16 gCOD·m ·d for 400 days. Dur- ing this experiment, FIB (up to 3.9 log10-units), bacteriophages (up to 2.7 log10- units), COD (up to 94 %) and turbidity (up to 97%) could be significantly re- duced. However, essential crop plant nutrients as such as nitrogen and phospho- rous, which are abundantly present in municipal wastewater and which is why wastewater is appreciated by farmers despite the health risk it poses, were not significantly reduced by the applied water treatment. Overall, the performance of the biochar AnBFs was significantly better than or equal to the sand and rice husk filters, and the effluent water quality was suit- able for unrestricted irrigation following the multibarrier approach of the WHO (2006) guidelines. This concept was validated in a greenhouse experiment, using the effluent of biochar AnBFs in comparison with untreated wastewater as irriga- tion water source for lettuce. Thereby, contamination with FIB of most of the 12 lettuce plants which were irrigated with treated wastewater were below the de- tection limit of 3.8 MPN per g fresh weight (MPN = most probable number) and thus more than 3 log-units lower than for untreated wastewater. Additional, soil contamination of treated wastewater pots were in the same range as tap water irrigated pots, while soils irrigated with raw wastewater showed a 2.5 to 3.5 log- units higher contamination. The results of all these experiments covered in this thesis provide a solid basis for the implementation of anaerobic wastewater filtration systems in developing countries, which can potentially contribute for safer food production and envi- ronmental protection. Acknowledgements Even though only my name appears on this work, numerous people have con- tributed to its creation in various ways. You all really deserve a big "thank you!" First of all, I would like to thank my doctoral supervisor Prof. Dr. Marc Wichern. Not only for his great mentoring and support in the preparation of this thesis during all these years, but also because he gave me the opportunity to work in an international and interdisciplinary research project. As I wrote this thesis and reflected on the last few years, I realized how much I had learned from him. It was a great time in Bochum. I am very grateful to Prof. Dr. Harald Horn who kindly accepted to review this thesis. Without my former professor, who raised my interest in water and wastewater treatment, I might never have done my doctorate at the Ruhr-Universi- tät Bochum. Furthermore, I would like to thank Prof. Dr. Daniel Balzani for his willingness to preside over the committee. I owe my gratitude to Dr. Manfred Lübken for his support in all aspects of this work. In particular regarding scientific questions, proof reading and accounting. Additionally, thanks to the German Federal Ministry of Education and Research (BMBF) for funding the UrbanFoodPlus project (Grant number: 031A242) and the overall project leaders Prof.