Wastewaters are not wastes

Chapter 4 in: "Living with water: targeting quality in a dynamic world"
Antonio Lopez
Istituto di Ricerca Sulle Acque, Bari, Italy

Wastewater sewers were already common in ancient Rome, but these had been built basically to remove foul-smelling water. The Romans probably were unaware that this was a wise thing to do, since it was not until the 19th century when populations had become so concentrated that outbreaks of life-threatening diseases were traced to bacteria in polluted wastewaters. The building of infrastructure and development of sewers began in London, introducing the solution to public health problems created by unsanitary conditions.

Since that time, the practice of wastewater collection and treatment has been developed and perfected. Today we are using a combination of technical, biological, physical, chemical, and mechanical techniques. As a result, public health and water quality are protected better today than ever before. While treatment is a necessary step to protect our environment or for sanitary reasons, it is potentially a sustainable process when recovery of resources will be implemented.

Wastewater treatment
The word wastewater implies that the result of water use is a waste product. Indeed, water serves as a transportation system for wastes from households, commerce and industry. Historically, wastewater treatments were aimed at removing natural pollutants such as organic substances and pathogenic microorganisms, a combination that forms a risk for human health. Over time and with increased human activities, other substances entered the wastewater streams: nutrients (nitrogen from agriculture and phosphorus from soaps) and novel chemical substances (solvents, pesticides, dyes, pharmaceuticals).

The reason to remove chemicals before discharging from the wastewater treatment plant is obvious, we would not want to harm the environment and neither ourselves. High concentrations of nutrients, especially phosphates and nitrates, also need to be avoided in water bodies, since these can promote excessive growth of algae. As algae die and decompose, other micro-organisms deplete the water of available oxygen, causing the death of a range of organisms, such as fish. This phenomenon is called eutrophication.

Reclaiming water
Water scarcity is a problem of growing importance and magnitude, in fact, because of many reasons, including climate change, water resources availability is progressively declining and nowadays freshwater resources are struggling to meet demand. increasing constraints on the development of new water sources have spurred a variety of measures to conserve and reuse water over the last two or three decades.

Worldwide, agriculture is the largest user of water and has accounted for 67% of total freshwater withdrawal in the world in 2000. The conservation of agricultural water through efficient re-use of effluent wastewater is essential for a sustainable water management. The reclaimed water that is intended for reuse should be treated adequately and monitored for its composition to meet the needs of the planned crop and prevent damage as a result of excess nutrients or pollutants.

In contrast to the seasonal nature of non-potable water reuse applications such as irrigation, a major benefit of using reclaimed water for industrial applications is that water use requirements are relatively constant year-round. A good example of water re-use in a car-wash is detailed in the schematics below.

Reclaimed water that replaces or supplements regular water sources in industrial activities sometimes needs to meet higher quality standards than in a car wash. A great impulse to the technique of reclaiming water was achieved during the last decade by developments in the area of micro-filtration. In this process water is forced through synthetic membranes with small pores, removing unwanted contaminants.

Urban usage
Most of the water used in households do not need to meet the quality of drinking water. Think of applications like toilet flushing, washing the car, garden watering, fire-fighting. If such a second source of less purified water would be used, this would mean that a second system of water supply would need to be installed. This is not common practice. Usually reclaimed water is either released into surface water or is recharged into aquifers and groundwater. Further purification is taken over by nature and the water may again be taken in by the same or another drinking water plant, resulting in indirect re-use of wastewater.

Direct re-use of wastewater has also been developed, which means that fully treated wastewater is directly added into the normal drinking water distribution systems. A well known system has been in operation in Windhoek, the capital city of Namibia, since 1968. In 2002 the plant was expanded to deliver 21,000 m3 per day of potable water that is directly used in the city.

Another example is Singapore, where the treated water from a water reclamation plant are further purified through subsequent processes of microfiltration, reverse osmosis, and UV radiation. This ultra clean water, known as NEWater in Singapore, is used by industries for wafer fabrication and air cooling purposes. A small percentage of NEWater (about 2.5% of the country's daily water needs) is blended with regular reservoir water before undergoing conventional treatment at the waterworks before being supplied to homes.

Recovery of nutrients
Not only water is worth reclaiming from wastewater: it has proven cost effective to recover phosphorus and nitrogen compounds as well.

In the case of phosphorus there are good reasons for doing this: i) phosphate rock resources are limited and declining both in quality and accessibility; ii) the growth in the world population leads to an increase in phosphate fertiliser consumption and iii) the pressure to remove heavy metals from all phosphate products (including fertilisers) derived from natural phosphate rock will lead to an increase of such raw material prices. Therefore it has become increasingly cost effective to recover phosphate from waste waters, that would be comparatively free from heavy metals.

Conventional methods of phosphorus removal from wastewater are based on chemical precipitation with lime, aluminium or iron salts. There is increased interest in biological removal using polyphosphate-accumulating bacteria under anaerobic conditions. Phosphorus will be enriched in the remaining sludge in this process. For the conventional removal of nitrogen compounds from wastewater, separate steps of nitrification (NH4+ -> NO3-) and denitrification (NO3- -> N2) are used. A new one-step anaerobic ammonium oxidation (ANAMOX) biological method is now also under development. Nitrogen gas escapes in the air, therefore eutrophication by nitrogen compounds in effluents is avoided.

Unfortunately these methods for the removal of nutrients can not be used for their recovery. A process, described 30 years ago, that is based on ion exchange steps and precipitation would however yield the compound struvite, or ammonium magnesium phosphate (MgNH4PO4). Economic evaluations indicate that, if compared with conventional technologies for removing nutrients from wastewater, the RIM-NUT process results an economically advantageous alternative and recovers more than 90% of the loaded nutrient species as a solid slow-release premium quality fertilizer.

Energy generation
Biomass, present in wastewaters, finds its origin in photosynthesis that needs solar energy to fix carbon dioxide (CO2) to form glucose. Biomass can in turn be used to produce energy: by a quick fire, or a slow biological process. In a process called 'anaerobic digestion', micro-organisms may convert organic substances into methane (CH4, biogas) that is a more suitable energy carrier for human usage and an alternative power source for cooking, lighting, heating and engine fuel.

Another technology that utilises organic substances and micro-organisms can even produce electricity. Here so called microbial fuel cells are anaerobic bioreactors where micro-organisms catalyse the conversion of organic molecules and the liberation of electrons. The movement of electrons towards a cathode in a chamber separated by a proton exchange membrane, generates electricity.

Wastewater can even be used as a resource for bioplastics. These biopolymers can replace conventional plastics, especially if these need to be biodegradable. The cost of preparing bioplastics under controlled conditions and by using pure microorganisms in culture fermentation processes is too high to be competitive to petroleum-based derived plastic materials. However, since polyhydroxyalkanoate (PHA) , an excellent source for bioplastics, is an intermediate metabolic product in activated sludge from certain stages of the wastewater treatment, the simpler facility construction and material recovery from wastes makes this process much more competitive. When combined with one or more opportunities for resource recovery, as describes above, wastewater treatment may in the future be profitable, instead of a cost to society.

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