Optimisation of sewage sludge treatment to foster dewaterability and nutrient recovery
Annually, about two million mega grams of dry solids (DS) of sewage sludge accumulate in wastewater treatment in Germany. According to the Statistical Federal Office (Destatis, 2013), 1,846,441 mega grams DS of sewage sludge were utilized in 2012. Besides incineration, the exploitation of sewage sludge in agriculture as fertilizer plays a significant role. In 2012, about 600,000 (544,065) mega grams of dry sewage sludge were applied on agriculturally or horticulturally used soils, which corresponds to 30.0 % of total amount (Bundesamt, 2011, AbfKlärV, 1992). Before disposing, the sewage sludge must be dewatered which is usually executed using synthetic flocculation aids like polyacrylamide (PAM) and its derivatives (Tuan et al., 2012). According to the fertilizer ordinance, as of 01/01/2017 20 % of all compounds as well as the end product of the used synthetic flocculation aids must be degraded two years after agricultural application (DüMV, 2012). Despite this regulation, an accumulation of polyacrylamide in the soil can be expected, since it is allowed to apply 5 mega grams DS of sewage sludge per hectare in 5 years (AbfKlärV, 1992). Seybold (1994) suggests that PAM is mostly resistant to microbial decomposition and mainly physically degraded. Not the PAM but its monomer (acrylamide) is known for its neurotoxicity (LoPachin and Gavin, 2012). Even if it does not permanently accumulate in soil, a potential toxicity exists. Therefore it is desirable to substitute the synthetic polyelectrolyte by a natural based and easily bio-degradable alternative flocculation aid. Several studies investigating the flocculation properties of cationic starch have been carried out (Rath and Singh, 1997, Khalil and Aly, 2001, Haack et al., 2002, Schwarz et al., 2006, Hebeish et al., 2010, Wang et al., 2013) but not in combination as nutrient incorporating fertilizer. The scarcity of phosphate rock sources has been an important issue in the last decades (Pinnekamp et al., 2007, Kabbe, 2013) as phosphate is a key nutrient for life on earth. As Kabbe (2013) states: “It is the key element in our genome, cellular membranes, skeleton and molecule adenosine triphosphate (ATP), the organism’s main energy storage.” Phosphorus is a non-substitutable nutrient in agriculture. An application of phosphorus fertilizer manufactured from non-renewable phosphate rock due to high crop yields is inevitable (Syers et al., 2011). Approximately 80 % of mined phosphorus are used for the fertilizer industry worldwide, in Germany even 85 % (Pinnekamp et al., 2007). Cordell et al. (2009) points out that the current global resources will be depleted in 50 to 100 years. For that reason, fostering of phosphorus recovery has become a current topic in the last years. So the European project P-REX – Sustainable Sewage Sludge Management fostering Phosphorus Recovery and Energy Efficiency was coined in September 2012. 16 European partners managed by project leader Dr Christian Kabbe from Berlin Centre of Competence for Water gGmbH work on 6 different working areas trying to close the phosphorus loop (shown in Fig. 1). Besides phosphorus recovery technologies from sludge processes or incineration ash out of sewage sludge, direct application on arable land is one branch of the phosphorus cycle. Within the working area 3 (WA3), the demonstration of applicability of green polymers for sludge dewatering step is one working package (P-REX, 2013). Within P-REX, this research attempts to combine phosphorus recovery and substitution of synthetic flocculation aids using natural based (green) polymers within sewage sludge treatment. The flocculation properties of a starch based cationic green polymer will be examined by measuring selected dewaterability indicators. Diverse sludge types from 3 different German wastewater treatment plants will be analysed after applying polyacrylamide/starch blends with defined ratios. As to phosphorus recovery, the phosphate contents of treated sludge water will be measured and a possible nutrient incorporation will be surveyed.