Taking the waters
Over the next decade, the UN hopes to bring safe sanitation to more than a billion people. They're ambitious plans, but Frank Rogalla seems to think they're attainable, with a combination of new technology and radical new thinking
Sustainability of water and wastewater services is one of the buzz-phrases of the United Nations' Millennium Development Goals (MGD) initiative, which aims to halve the proportion of people lacking access to sanitation by 2015. While it is estimated that more than one billion people lack access to safe water, more than 2.5B - half the population of the developing world - lack safe sanitation.
However, the cost of achieving the sanitation goals with conventional means would be staggering. For instance, Dublin's Ringsend WwTW, designed for a population equivalent (PE) of 1.7M, cost £200M or £66/PE. Higher costs are often observed if complete nutrient removal and tertiary treatment is included, as at Reading, where the investment cost of £80M covers effluent treatment for 300,000 PE and solids treatment for 350,000 PE, making it four times more costly than Dublin. A new underground WwTW in Turku, Finland, is even projected at a cost around £110M for a population of 280,000 PE, which at £385/PE is five times higher than Dublin.
Furthermore, this cost is only for the end-of-pipe treatment plant, not including domestic equipment or the collection system, which can add up to 80-90 % of a typical infrastructure cost. A recent estimate by the German Water and Wastewater Association (DWA) estimated that, in Germany, about £275/PE is needed just to bring the existing run-down sewer network back into shape.
The irony is that, with conventional sewerage, available fresh water is captured at great expense, treated to levels of ever-increasing purity, only for the greater part of the flow to be used to flush human contamination into the sea by the shortest pathway, dispersing pollution in rivers and marine environment. Does this mean, as Professor Peter Wilderer asserted on accepting the 2003 Stockholm Water Prize, that the centralised sewer was the greatest sin of the sanitary engineer?
To reduce cost and increase the reuse of resources, new approaches are being proposed, such as source separation of nutrients to avoid the mixture of water, urine and faeces. In principle, each human produces enough nutrients to grow the cereals needed for his own nutrition. It would make sense to capture those nutrients at source and reuse them, rather than flush them down the drain to cause eutrophication, and then buy artificial fertilisers. In India, it is estimated that the excretion of a family of six would produce a yearly fertiliser quantity of 75kg.
The answer that lies within us
To capture these nutrients, ecological sanitation toilets are being proposed - over a million are said to have been installed in China over the last five years, at a cost of £27.50 per unit. While this is double the cost of more conventional toilets, such as VIP latrines or single-pit pour-flush toilets, the latter risk contributing to the deterioration of the shallow groundwater which
is often used for drinking water.
The majority of the nutrient value from human waste derives from urine. If there was an economical way to capture the liquid waste separately from the solids, about 80% of nitrogen and half of the phosphorus would be captured at the source. Nutrient removal in WwTWs would thus be mostly unnecessary, as the normal biological uptake during treatment would fix the remaining nitrogen and phosphorus in the biomass.
Even in the EU, there are still significant proportions of population not connected to a central treatment plant. Levels are thought to be around 40% in Greece, more than 50% in Belgium, Portugal and Slovakia, and close to 70% in Bulgaria, Hungary and Iceland.
One new concept for decentralised treatment in developed countries is to separate urine, which would be stored and precipitated as struvite (magnesium-ammonium-phosphate) by addition of MgO. The remaining wastewater could be treated with membrane bioreactors (MBR), allowing for reuse for toilet-flushing and reducing freshwater consumption by around 33%. Additional benefits could be gained from irrigation of the reuse water, especially in regions with dry summers such as eastern and southern Europe.
Just such a process, incorporating an MBR, has recently been tested on a family of four living in a house in Solothurn, Switzerland. During the test period, the solids production was limited to around 0.17g COD/g COD removed, corresponding to about 100g TSS/d, and was removed in a filter bag at about 10% solids.
Despite the low solids production, significant phosphorus uptake was observed, with a content in the solids of 0.05g P/g COD or 0.7g /PE/d, i.e. more than twice the value expected through normal biological uptake. The total phosphorus concentration was thus reduced from 20-30mg/l to about half that.
Nitrification in the low-loaded reactor was complete, and total nitrogen removal from an influent concentration of about 100mg TN/l was around 70% after adjusting the aeration sequence to favour denitrification. As well as the conventional parameters, colour is of significance for reuse, as it is visible as soon as a water depth of a few centimetres is stored. Several tests to remove colour showed a reduction of around 70% by activated carbon adsorption, ozonation or chlorination.
What goes in, what comes out
One key element to sustainability is energy use and CO2 emissions. Conventional
WwTWs for nutrient removal need around 5-10kWh/PE/d, including the reuse of digestor gas, or around 0.05kwh/m3 for a water flow of 200 l/PE/d. Because of the high recycle, coarse bubble aeration to clean membranes, and less O2 transfer at high biomass concentrations, the energy consumption of large-scale MBRs are estimated at somewhere in the region of 0.5-1.2kWh/m3, although recent improvements with stacked membranes or air-cycling reduces that to about 0.3kwh/m3 - still more than five times higher than conventional systems.
Investment-wise, it is far from certain that the system will be competitive with centralised approaches, which for large systems are somewhere in the order of magnitude of £82/PE/year, or up to twice that value for smaller collection and treatment systems.
One intermediate approach to preserving the benefits of centrally planned and operated sewer systems while reducing their cost is the so-called 'simplified' or 'condominial' sewerage, as developed in Brazil in the early 1980s, and reflecting the recommendations of back-yard sewerage in the UK in the mid-19th century (General Board of Health, 1852, Mara, 1999).
The shallow pipes of Brasilia
Sewers are laid within the boundaries of private properties (condominia) rather than in public roads, and smaller-diameter pipes (100-150mm) are laid in shallow ground (less than 500mm), which allows easy access through chambers rather than manholes. The capital, Brasilia, now sees this solution as standard for rich and poor areas alike, and has implemented more than 1500km of pipes - at an average cost of £14-28/PE, less than half that of conventional systems, and in the same order of magnitude as onside systems, as long as population density is higher than 200PE/ha.
The downside is that treatment still has to be provided in a central location. To reduce cost, energy need and solids production, anaerobic treatment has been largely adopted in tropical countries as the first treatment step. Conventional anaerobic bioreactor configurations, such as the upflow anaerobic sludge blanket (UASB) reactor, have been applied for treatment of municipal wastewater since the 1980s. For instance, in Brazil's Recife Metropolitan Region, a 810m3 UASB reactor treating municipal wastewater was evaluated over a 30-month period in the late 1990s (Florencio et al., 2001). It was determined, in the treatment of degritted municipal wastewater, that at COD loadings up to 2.5kg/m3, a COD removal of 80% or higher could be achieved.
This performance level is not yet acceptable to allow for direct discharge, due to the presence of a significant concentration of organics (e.g. soluble COD approximately
85mg/l) and particulates in the effluent (i.e. TSS approximately 80mg/l). Therefore, additional treatment is proposed, such as:
One further possible treatment alternative would be to include an anaerobic MBR to
improve effluent quality in the treatment of municipal and low-strength industrial wastewaters. The increased biomass levels achievable with this technology could even help to extend anaerobic pre-treatment to countries with moderate and colder climatic conditions. Although to date there are no large, commercial-scale anaerobic systems treating municipal wastewater or low-strength industrial wastewaters at low temperatures, research indicates that performance comparable to tropical conditions can be expected, albeit at slightly lower treatment efficiency.
There is, therefore, vast interest in the anaerobic MBR technology, and recent development activities have taken place in Europe and the Far East. Full-scale application has generally been limited to enhancing performance of conventional anaerobic digesters treating municipal wastewater sludges (Pillay et al., 1994), and application of anaerobic suspended growth MBRs for treatment of sludge and slurries from livestock farming operations (Norddahl and Rohold, 1998).
A few full-scale anaerobic MBR systems treating wastewaters were in operation in the 1990s, e.g. the ADUF system at the Meyerton corn maize mill in South Africa (Ross et al., 1994).
One important factor to consider with anaerobic reactors is the emission of greenhouse gases which contribute to global warming. Whereas aerobic conventional treatment emits CO2, both because of its high-energy consumption and as a product to the bioreaction, anaerobic reactors emit methane. If the biogas is not carefully captured and burned, or used for energy regeneration, transforming it to CO2, methane will remain in the atmosphere for approximately 9-15 years - and it is over 20 times more effective than CO2 at trapping heat in the atmosphere over a 100-year period.
Standards of sludge viscosity
Regarding the matter of sludge characteristics and oxygen transfer, the sludge characteristic differs from conventional activated sludge, mainly due to the higher MLSS. The sludge viscosity increases with increasing MLSS. The viscosity of the MBR sludge is non-Newtonic i.e. it decreases at higher shear stress (Cornel et al., 2002). The higher viscosity may lead to a lower ±-value (= ratio of the aeration coefficient kLa under process condition to the clean water aeration coefficient), which is about 0.5 ± 0.1 at MLSS content of about 12g/L.
As for the membranes, the design of their surface area is important to the economic efficiency of the process. The flux depends on the membranes, the modules, the trans-membrane pressure, the wastewater composition, and on fouling/scaling. For the design the net flux is the important parameter which characterises the overall flow rate including breaks and back flushes. For industrial wastewater in general, pilot tests have to be performed. The resulting flux is often as low as 10-15 (20) L/(m2.h) for immersed membranes and up to 120 L/(m2.h) for tubular membranes.