NaWaTech Approach

At present, urban settlements in developing countries are growing five times as fast as those in developed countries. As a consequence, today the urban population has exceeded the rural population of the world. In developing countries, urban population is predicted to grow from 1.9 billion in 2000 to 3.9 billion in 2030 (BROCKERHOFF 2000). India is projected to become the most populated country by 2030, with an estimated population of 2.53 billion inhabitants (CENSUS 2001). About 30 % of India’s people (almost 300 million) live in cities and towns generating over 60% of the country’s GDP and 90% of government revenues (IGOVERNMENT 2008). The rapid urbanisation and population growth and the lack of appropriate sanitation and wastewater treatment facilities result in water shortage, degradation of river, streams and aquifers and over exploitation of groundwater resources.

It has been demonstrated that the conventional approach to water management has serious inefficiencies, such as providing high quality drinking water for all domestic purposes, large piping systems difficult to construct and maintain, large quantities of drinking water to transport human excreta, dependency of extensive energy supply for advanced treatment system, production of large quantities of sludge and loss of useful elements with the sludge (e.g. phosphorus). It is thus clear that current models of urban water management and their corresponding infrastructure have already failed or are on the verge of collapse from the perspective of cost-efficiency, performance and sustainability.

In order to optimise the operation, and maintenance and technically cost-effectiveness of future urban water management systems, there must be a shift of paradigm from an approach with centralised mixed end-of-pipe solutions to the design of integrated urban systems. Such systems should optimise water use and reuse, minimise operation and maintenance and be flexible.

Facing the challenges of future urban water management, natural water treatment show a high potential due to their flexibility and adaptability, cost-efficiency, and low requirements for building, operation and maintenance and energy supply. Natural treatment systems refer to systems that use natural capacities in soil and vegetation to absorb and retain water, and to take-up, transform, or otherwise treat pollutants in water.

Natural systems are much more convenient than the conventional wastewater plants during the operational phase, because they require less energy than conventional systems. Limited mechanical devices are used in these systems thus reducing the maintenance costs. In addition it was found that natural systems are generally efficient for the removal of most of pollutants. Finally and most importantly these systems are found to be very reliable even in extreme operating conditions. They can better absorb a variety of both hydraulic and pollution shocks and have the capability to adapt to new, different, or changing requirements, hence making them more robust and resilient systems.

Natural treatments can be integrated in the urban scenario as flexible small-scale decentralised measures, such as infiltration devices or constructed wetlands for stormwater control and managed groundwater recharge. Thus, urban natural water treatment systems have a wide range of socio-economic and ecological services, including improved public amenity and health, flood retention capacity, groundwater recharge and drinking water supply.

In Europe, those systems have been developed for many years and their potential for the application in developing and newly-industrialised countries is widely accepted. However, the location of India and many developing and newly industrialised countries in warmer climatic zones sets different environmental conditions.

On the basis of a detailed inventory of natural treatment systems and compact treatment options, several promising axes have been identified:

• Wastewater and stormwater treatment and reuse for the managed aquifer recharge (MAR) (constructed wetland; SBRs & MBRs, soil aquifer treatment and aquifer storage and recovery);
• Stimulation of water retention and self-purification capacity of water resource via in-stream remediation using eco-hydrology principles;
• Improvement of surface water quality via bank filtration (lake or river bank filtration) for the generation (indirect) potable water;
• Secondary treatments for drinking water (sand filtration; membrane filtration; UV disinfection).

Previous experiences have shown that soil aquifer treatment (SAT) is a promising technology for the reclamation of stormwater and managed aquifer recharge. SAT technology involves infiltration of stormwater or secondary effluent (pre-treated wastewater) through a recharge basin with subsequent extraction through recovery wells, and embodies both treatment, dominant in the vadose (unsaturated) zone, and storage within the saturated zone (aquifer) (AMY & DREWES 2005). SAT in combination with both, advanced wastewater treatment system (e.g. activated sludge) or natural systems (e.g. constructed wetlands) has proven to be effective in many MAR sites (MELIN 2009; KAZNER 2009). However, little is known about the parameters controlling them and implementing man made technologies for enhancing those attenuation processes is not yet a common practice.

Constructed wetlands (CWs) are low cost eco-technological wastewater treatment systems that in the past 30 years have been set up all over the world as an alternative to conventional mechanical intensive systems for the treatment of a wide range of wastewaters like municipal, industrial, urban run-off, leachate, agricultural, etc. CWs can remove a variety of substances (organic matter, nutrients, microbiological contamination, micro-pollutants, etc.) and have been used worldwide for a large number of applications (GHERMANDI et al. 2007; ROUSSEAU et al. 2008; LLORENS et al. 2009; SEGUI et al. 2009) including as a pre-treatment step before artificial recharge of groundwater bodies in China (GROSSE et al. 1999; PERFLER et al. 1999). They have been shown to be adapted in particular to urban areas for the small-scale decentralised treatment and storage of stormwater as well as the treatment of greywater or blackwater prior to SAT.

Bank filtration (BF) consists in the abstraction of water from aquifers that are hydraulically connected to a surface water body, which is in most cases a river system. It is based on percolation of the surface water through the ground into the aquifer enhanced by the pumping in drinking water wells close to the riverbank. During the percolation processes potential contaminants from the surface water are removed by filtration, adsorption, reduction and biodegradation (RAY et al. 2002; TUFENKIJ et al. 2002; ORLIKOWSKI et al. 2006). The capacity of self-purification depends mainly on flow velocity, hydraulic residence time and the covered distance determined by the permeability and the hydraulic potential in the aquifer. In Central Europe, BF has been a traditional, efficient and well-accepted method of surface water treatment for more than 100 years for public and industrial water supply (GRISCHEK et al. 2002; SCHMIDT et al. 2003; SHARMA & AMY 2009). Due to the easy implementation and little maintenance, an effective removal of pathogens, solids and toxins of algae from surface water can be achieved. Because of its capacity to buffer shock loads and extreme climatic conditions (i.e. floods and droughts), BF has been suggested to be a useful drinking water treatment for developing and newly-industrialised countries.

For India situated in tropical, semi-arid and arid climates zones, little is known about the performance, operation and maintenance of natural water treatment and infiltration system. Especially the higher temperature and organic solid load in the water is a parameter expected to substantially affect the sorption and biodegradation process in soil filtration systems due to lower oxygen content in the water and thus changing redox conditions.

Sequencing Batch Reactors (SBR) and Membrane Bioreactors (MBR) represent intensive water treatment systems that are able to effectively treat heavily contaminated water sources as stand-alone systems or in combination to the above mentioned natural, extensive systems. SBRs are a variation of the well-known activated sludge system where the natural degradation and conversion processes of water bodies are utilised. In contrast to the natural treatment systems presented above, SBR and MBR are designed to concentrate biological degradation on a small footprint enabling high performance by control of the different biological treatment processes. The advantages of SBR and MBR over natural systems like e.g. constructed wetlands lie in contrast to increased efforts that are required for operation. Beside investments in technical equipment the active aeration and filtration (in the case of MBR) result in higher energy demands, hindering the application for financial reasons or simply because of lack of power supply in many regions of the world. The maintenance of membranes (cleaning and replacement) also leads to increased costs and efforts. Last but not least, both systems require sufficiently trained personnel for operation and maintenance.

As reaction to the main shortcomings of these technologies, research and development has been concentrated on reduced energy demands and lower maintenance requirements. Especially when it comes to the applicability for developing countries, adaptation is needed for applicability. Low energy membranes, hydrostatic filtration and simplified reactor designs as well as the combination to anaerobic treatment have been successfully applied in various projects. Having the various water related problems of India in mind (rapid urbanisation, high population density, contamination of surface and wastewaters with various toxic compounds) the implementation of adapted SBR and MBR treatment (or combined as SMBR) is a vital part of the NaWaTech concept. For NaWaTech the potential application of SBR and MBR covers and wide range: from decentralised treatment systems to components of the recharge chain. The possible reason for their inclusion can be demand for water re-use at high quality requirements in the urban context, the improvement of the pre-treatment for BF or SAT or the targeted removal of trace contaminants (micro pollutants) that would pass the other stages. MBR systems have wide applicability for municipal and industrial wastewater treatment but also for agriculture, food production and even for potable water treatment. Hence, in the scope of NaWaTech SBR and MBR are seen as complementary components of the natural systems to enlarge the modular flexibility.

NaWaTech stands for “Natural Water systems and treatment Technologies to cope with water shortages in urbanised areas in India”. As an integrated approach, NaWaTech is based on the following axis:

• Interventions over the entire urban water cycle, which includes water sources, purification, distribution, use, collection, treatment and reuse.
• Optimisation of water use, by diminishing water use at home, reusing wastewater and preventing pollution of freshwater source;
• Prioritisation of small-scale natural and technical systems, which are flexible, cost-effective and require low operation and maintenance.

As shown in the Figure 2.5, the concept is based on optimised use of different urban water flows by treating each of these flows via a modular natural system taking into account the different nature and degree of pollution of the different water sources. Thus, it will cost-effectively improve the water quality of urban surface water and restore depleting groundwater sources.

Due to the multi-barrier approach, these systems are also able to treat heavily polluted water in order to reuse them and to supplement traditional sources to cope with water shortages today and in the future. The collected waters are treated in different steps by natural and technical processes at the surface or underground before they are reused to supply the urban population:

• Stormwater is collected and pre-treated in constructed wetland before being filtered through the soil and can be stored in the aquifer.
• Blackwater is treated anaerobically (producing energy and biogas) before being treated and collected in constructed wetlands together with or without the greywater. The effluent of these constructed wetlands will be either discharged to the surface water (if removal efficiency is satisfying) or reused for the urban agriculture or landscaping.
• Rainwater is collected on rooftops and either directly reused or fed into the aquifer.
• Surface water quality will be enhanced by in-stream and in-lake eco-hydrological systems prior to sub-surface passage and re-extraction for the urban water supply.

In addition to that, a low-cost and viable post-treatment option could be used based on different steps (sand filtration, low-pressure membrane filtration or UV disinfection) depending on the required quality for service or potable water.

A NaWaTech system is integrated as green infrastructure into the urban scene providing additional socio-economic benefits including flood absorption and buffering of seasonal variability in the availability of water, recreational values for the urban population, space for animals and plants and improved air quality.

The NaWaTech approach: every available source is reused locally minimising the dependency from external freshwater sources and minimising the pollution of downstream water users.

This holistic approach minimises the urban water footprint and enhance the water security of the area, as the water cycle is closed at a local level. It also minimises the pollution of ecosystems and water sources for downstream users, as almost minimal amounts of freshwater get polluted and polluted water is treated and reused locally.