Elsevier

Bioresource Technology

Volume 100, Issue 23, December 2009, Pages 5537-5545
Bioresource Technology

Review
Maximum use of resources present in domestic “used water”

https://doi.org/10.1016/j.biortech.2009.05.047Get rights and content

Abstract

Environmental protection and the sustainable management of natural resources stand at the foreground of economic and technological activities worldwide. Current sewage technologies, however, deal with diluted wastes and do not focus on recovery and are therefore not sustainable. Here, the most promising methods available for the recovery of nutrients (nitrogen, phosphorus), organic material and energy from “used waters” are examined both at the decentralised and centralised level. Novel approaches for water processing, not implementing aerobic biological treatment as a core technology, are conceived and critically evaluated regarding efficiency, diffuse emissions and requisite costs. By implementing up-concentration of dilute wastewaters, the concentrated stream becomes suitable for the waste-to-energy strategy.

The approach of up-concentration of municipal effluent at arrival at the water treatment plant followed by anaerobic digestion of organics and maximal reuse of the mineral nutrients and water is estimated to have a total cost of the order €0.9/m3; the latter is comparable to that of conventional aerobic treatment technologies which has little or no reuse. It is argued that in view of the fact that recovered nutrients will become of increasing economic and ecological value, this new conceptual design for the treatment of “used water” will become feasible in the next decade.

Introduction

Throughout the world, water scarcity is being recognized as a present or future threat to human activity and as a consequence water reuse strategies should deserve major attention (Fritzmann et al., 2007).

Water scarcity has become a global issue and not only a problem relevant to arid zones. Continuous population growth, rising standards of living, climate changes, industrialization, agriculture and urbanisation has resulted in water becoming a limiting resource. This scarcity is often the limiting factor for economic and social development (Singh, 2007). According the United Nations predictions, between two and seven billion people will face water shortages by the year 2050. Even today about 80 countries, comprising 20% of the world population are suffering from serious water shortage (United Nations, 2006). Also in countries with the availability of high quality water, industry and agriculture have to compete for these resources with the households. Due to the increasing pressure on the use of groundwater in the last decades, water industries have to look for alternative water resources which have led to the implementation of closed cycle processes in domestic and industrial water supply (Dewettinck et al., 2001, Verdickt et al., 2007, Hoeijmakers et al., 2007). In these industries, wastewater treatment is regarded as an integral part of the production process rather than an end-of-pipe solution.

Effluents, originating from domestic wastewater treatment plants, deserve a special attention because of the availability at the place where water reuse strategies should be adopted, i.e. urbanized regions. This water resource is able to provide up to 80% of the need of freshwater (Qin et al., 2006).

The initial goal of wastewater treatment was to protect downstream users (Wilsenach et al., 2003) and in the last decades environmental protection came into the picture by the stringent effluent standards for nutrients. Yet, one cannot overlook the fact that the conventional approach brings about diffuse emissions such as CH4 (Guisasola et al., 2008) and H2S (Zhang et al., 2008a) in the sewer and N2O in the aerobic treatment system (Colliver and Stephenson, 2000). The focus on nutrient removal has as a result that the costs for wastewater treatment in regions with sensitive surface waters is dominated by the conversion and elimination of nitrogen and phosphorus. Since these effluents are the perfect source for the abundant demand of high quality water, domestic wastewater treatment can be seen as a part of the freshwater production. This incorporation has a high impact on the environmental footprint correlated with freshwater production. As an example hereby, the Dow’s Benelux site at Terneuzen (The Netherlands) is recently reusing the local community’s treated wastewater. The effluent of the sewage treatment plant is subjected to membrane filtration. The product of the latter is implemented by the industry to generate steam. By this, three million tons per year of water previously discharged into the North Sea after just one use are now recycled. Reusing this water results in 65% less energy consumed at the facility – compared to desalination of the same amount of seawater. The latter is equivalent with a decrease in CO2 emission of 5000 ton on a yearly basis. Next to CO2, also the chemical demand for the overall water supply process is significantly decreased (Baker, 2008).

Since high quality freshwater can actually be produced form wastewater, an extra effort for harvesting other resources, such as nutrients and energy from wastewater also should be considered in order to make the overall sewage treatment more sustainable. Indeed, besides a freshwater resource, domestic wastewater is also an important carrier medium for nutrients in the nutrients cycle. Nitrogen is an abundant element in the human’s diet. This results in a central position of man in the anthropogenic nitrogen cycle (Mulder, 2003). The supply of protein food is largely dependent on the anthropogenic atmospheric nitrogen fixation by the Haber–Bosch process. The generation of ammonia from the air requires 35–50 MJ per kg N in the form of fossil fuel for energy supply (Maurer et al., 2002). The potential of domestic wastewater to decrease the amounts of atmospheric nitrogen that have to be converted to ammonia fertilizer is substantial. Based on an average excretion of 13 g N per capita per day, the annual excretion is 4.75 kg N per capita. Research showed that about 30% fertilizer-N ends up in the domestic wastewater (Bleken and Bakken, 1997, Mulder, 2003). Hence, recovery of nitrogen present in domestic wastewater is able to cover some 30% of the current agricultural N demand. Time has come to recycle the N present in sewage rather than “wasting” it by nitrification and denitrification. This will allow minimizing the anthropogenic production of fertilizer.

Besides nitrogen, phosphorus is also present at substantial levels. Phosphorus is gained from rock phosphates, which are a limited resource concerning quantity and quality. The known rock phosphates deposits in the world are sufficient for 100–1000 years, depending on the efficiency of resource use during P fertilizer production and on the use of fertilizers in the next decades (Tinker, 1977, Smil, 2000, Zhang et al., 2008a, Zhang et al., 2008b). In order to give the phosphate industry and agriculture a sustainable future, it has been advocated that phosphate should be recycled (Driver et al., 1999). Furthermore, mining of phosphate has a heavy environmental impact. The production/mining of 1 kg P fertilizer leads to 2 kg gypsum which is contaminated with heavy metals and radioactive elements and is often not disposed of in an environmental friendly way (Driver et al., 1999, Wilsenach et al., 2003). The sources of phosphate pollution are agriculture (through the use of fertilizer), sewage and industry. Based on an average excretion of 2 g P per capita per day and addition of P originating from detergents, food waste, food additives and other products, a significant amount of the P ends up in the domestic wastewater. The first major concern is to remove the P in order to protect surface waters. By implementing P reuse strategies the need for commercial phosphorus fertilizers can be decreased. However, since wastewater is a heterogeneous and complex matrix of different elements, harvesting phosphorus from this kind of systems poses difficulties (Kvarnström et al., 2003). The poor bio-availability of P for plants and the contamination of the recycled P with heavy metals and organic micropollutants constitute a major challenge (Ito et al., 2008). One should therefore aim at technologies which minimize the level of contaminants associated with the phosphorus fraction.

Based on a series of inquiries with the field of practice, a new concept of dealing with “used water” is proposed, which is in sharp contrast to the one which revolutionized sewage treatment in the past century. Indeed, at current market prices, a potential of €0.35/m3 of resources can be recovered by appropriate techniques (Table 1). The latter value is mainly due to the value of the water as such, followed by the nutrients. The price of phosphate fertilizers has linearly increased during the last 5 years and the current price of processed phosphate rock is expected to be around €0.70/kg P. According the US Geological Survey, the price for unprocessed phosphate rock was €0.54/kg P in 2008 (Jasinski, 2007, US Geological Survey, 2008). The prices of nitrogen fertilizer have recently also become substantial (approximately €0.21/kg N) (US Geological Survey, 2007). Clearly, the time to redesign sewage treatment in a matter to maximize the reuse in the line of the cradle-to-cradle concept (McDonough and Braungart, 2002) has arrived.

This paper describes the cradle-to-cradle concept for centralised and decentralised systems incorporating the current environmental concerns. Whether or not decentralised systems are part of tomorrow’s solution for problems associated with dilute wastestreams and sewerage, focus must be placed on the minimization of diffuse emissions. The degree of valorisation of the present resources in the decentralised wastewater is depending on the scale of the installation, therefore, construction of small decentralised sanitation units should be limited for households for which sewerage connection with the treatment plant is not an option due to the high sewerage costs. In these systems the maximal recovery of resources should be coupled to minimization of diffuse emissions. For larger scale decentral sanitation units, the focus can be fully placed on maximal use of the resources in the form of electricity and heat originating from the anaerobic valorisation of the solids present in the wastewater. For centralised facilities, the implementation of nutrient recovery methods is essential. In this paper, we review the current practices of decentralised (Section 2) and centralised (Section 3) wastewater treatment, and propose new wastewater technologies based on concentrated wastewater, which allow maximum resource recovery.

Section snippets

Process concepts for decentralised sewage treatment

The current approaches for decentralised treatment of sewage treatment are well known. The often used septic tank represents an investment of about €3000 per family. Due to the anaerobic conditions, it converts a major part of the organic matter to methane gas (estimated at 20–40 m3 per IE per year) which dissipates into the atmosphere and thus contributes to the global warming (Vincke and Verstraete, 1999). Moreover, since decentralised treatments most often release the N and P in the form of

Water

Due to the presence of micropollutants, direct reuse of effluent from the conventional activated sludge (CAS) as drinking water is not feasible. An extra polishing step by membranes is necessary. After CAS, the effluent is treated by microfiltration (MF) or ultrafiltration (UF) so that a complete retention of the suspended and colloidal particles occurs. At the same time a major part of the pathogens is also removed. To counter the unwanted presence of organic micropollutants in the source

Discussion

The anaerobic digestion of up-concentrated sewage can be self-supporting. Indeed at a COD level of 5 g/L onwards, the biogas produced can cover the overall heat input costs (Thaveesri et al., 1995). In Table 3, the total costs associated with this new design for centralised water treatment are estimated. The approach of up-concentration of municipal effluent at arrival at the water treatment plant, with anaerobic digestion of all organics results in total costs of the order of €0.66–0.95/m3.

Concluding remarks

The conventional activated sludge process coupled with MF/UF and RO is complex and costs of the order of €0.793/m3 sewage treated (Fig. 2), while N and P are generally “wasted”. The key to new sewage treatment is the separation at home respectively the non-dispersion of resources at arrival at the treatment plant. The concept of up-concentration of municipal discharges, either at home or upon arrival at the water processing plant, enables fractionation of different components and recovery of

Acknowledgements

This work is in part financed by the project Sewage Plus 180B12A7 (MIP-project, Milieu- and Energietechnologie – Innovatieplatform, Berchem, Belgium). The critical reading and constructive suggestions of Marta Carballa, Ma Jingxing, Ilse Forrez, Tom Hennebel, David van der Ha and Roselien Crab were highly appreciated.

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