Enhancing wastewater collection treatment: Part 1

Enhancing wastewater collection treatment

Extracted from United Nations World Water Development Report 2017
Examining a number of options and responses for enhancing wastewater collection and treatment, with a special emphasis on the advantages of low-cost decentralised systems.

Sewers and waterborne sanitation

The importance of sewerage as a means of transporting waste materials away from human sources and other economic activities is well documented, as are its impacts. Despite more ecologically acceptable alternatives, waterborne waste disposal remains the prevalent method for sanitation and for evacuating wastewater from domestic, commercial, and industrial sources.

Other sanitation options, such as on-site systems, are perfectly suited to rural areas and low population density settings, but are expensive and nearly impossible to manage in dense urban environments, aside from the most developed economies. In many cases, significant challenges still exist in the collection and transport of faecal sludge from on-site facilities.

According to a study in the city of Kampala, more than 80% of the users of such facilities had no experience with emptying personal latrines and over 60% of the collected septage came from institutional and commercial sources. The number of households connected to sewer systems correlates (to a greater or lesser extent) with the connections to a water supply, although always in much lower proportions.

Various reports clearly show that, globally, the proportion of people connected to a sewer system (60%) is higher than had been previously assumed. Even in rural areas where the number of connections is typically low, there is a significant share of people with a connection to a sewer system (16%). This contradicts previously published estimates which quoted 10%.

Many large cities in developed and transitioning economies have extensive sewerage systems, some of which are still functioning effectively some 100 years after construction. London still relies on trunk sewers constructed in the Victorian era as part of the reticulation used today. Complications arise with increasing urbanisation and excessive connections to sewer systems that surpass their original design capacity.

Ageing wastewater collection systems generate a number of problems, including corroded concrete, cracked tile, collapse, and clogging. Addressing these problems can be expensive.

Low-cost sewerage

Driven by the high costs of conventional sewerage, methods of low-cost sewerage were conceived in response to the challenges faced by most developing countries: low tariffs combined with insufficient governmental budgets, high poverty, and expensive infrastructure. These low-cost systems come in many different types, but they normally use piped networks with a smaller diameter, laid at shallower gradients and at shallower depths underground. The design principles differ from those used in conventional sewer design and also focus on the concept that solid-free sewage is conveyed in the system, with interceptor boxes (similar to small septic tanks) collecting raw wastewater from a household or group of households.

These systems lend themselves to community management and are also very well suited to extend and expand existing systems. One drawback is that they are not suitable for stormwater drainage. The concept was first pioneered in Brazil. Low-cost sewerage systems have become a method of choice for neighbourhoods of all income levels, as they have all the characteristics required to be the de facto standard for all sewerage. However, conservatism among public health authorities and sanitary engineers has resulted in only sporadic uptake worldwide.

At this point, there are not yet many examples of the technology being evaluated, but the cost data, particularly from countries like Brazil, clearly show that it can be financially sustainable. In Brazil, the cost of simplified sewerage (a type of low-cost sewerage) per person has been shown to be twice lower than the cost of conventional sewerage.

Combined sewerage

One important issue in relation to wastewater collection is its source. In old systems, like the one used in Paris, the original sewers (from 1852) were designed only for rainwater and grey water; a later decree from 1894 imposed the house owners to put all kinds of wastewater, including blackwater, in the combined sewers.

Although a variety of users connect to sewer networks, most systems were designed as so-called ‘combined systems’, in which stormwater and other types of urban runoff are discharged to the sewers. This was done, presumably, in order to limit the costs of purchasing large diameter drains, but it resulted in dilute sewage in periods of high rainfall. Although this may have been acceptable when population densities were low and the assimilative capacity of the receiving waters was adequate, recent development and city expansions have led to a complex and often hazardous combination of different chemical and biological substances.

Combined sewerage should therefore generally not be considered an effective solution. In an effort to move away from combined systems, much work has been undertaken on sustainable urban drainage systems (SUDS). Sewer systems are suitable for so-called ‘point sources’ of pollution, but the real challenge is how to collect diffuse or non-point pollution sources.

Two major sources are the runoff from agricultural land that has received fertilisers, and the runoff from areas where intensive livestock is kept, as this often leads to drugs used for veterinary purposes being present in the water. Although many intensive agricultural facilities install collection and treatment systems, this is still not general practice due to the high costs associated and/or the lack of regulation or enforcement.

Many large cities in developed and transitioning economies have extensive sewerage systems, some of which are still functioning effectively some 100 years after construction.

Decentralised treatment (DEWATS)

India


Workers constructing a decentralised wastewater treatment system in India. Image: YouTube


In addition to centralised wastewater treatment plants, decentralised systems have also shown an increasing trend. Many of the approaches to decentralised wastewater treatment systems (DEWATS), pioneered by organisations such as the Bremen Overseas Research and Development Association (BORDA) and the Consortium for DEWATS Dissemination Society, have found their rightful place as a part of sanitation systems for rapidly expanding urban areas and also for certain isolated communities where conventional sewerage is precluded on economic grounds.

DEWATS and low-cost sewerage are naturally complementary. DEWATS can also serve as a medium-term solution pending the large-scale design of centralised systems, and there is significant flexibility on their use. Indeed, large-scale centralised wastewater treatment systems may no longer be the most viable option for urban water management in many countries, due to high maintenance costs and resource needs.

Moreover, they often require large areas of land and are too inflexible to meet the needs of rapidly expanding urban areas. This holds true for water supply and wastewater infrastructure, rainwater collection and drainage. DEWATS serve individual or small groups of properties. They allow for the recovery of nutrients and energy, save freshwater and help secure access to water in times of scarcity. They may require less upfront investment than larger, centrally piped infrastructures, and are more effective in coping with the need to scale up (or down) services to needs. However, they do require individuals with a minimal amount of training to take care of their operation and maintenance.

Through decentralised technologies, sustainable neighbourhoods in cities could partly replace traditional public systems. A challenge of DEWATS may be thatreedbeds local communities need to accept that they live close to the treatment facilities, so efforts must be made to make the plants aesthetically acceptable. For this reason, systems based on reed beds are often favoured.


Reed beds are often included in both centralised and decentralised treatment systems. Image: Biomatrix Water


Decentralised stormwater management

Decentralised stormwater drainage has a good potential for ‘source control’ technologies that handle stormwater near the point of generation. For instance, green roofs or pervious surfaces capture rainwater before it runs onto polluted pavements and streets. These solutions can alleviate peak flows, minimise the risks of urban floods and pollution, and reduce the need for investments in additional hard infrastructure and treatment facilities. They can attract private investments, encouraging property and land developers to invest in new buildings equipped with localised drainage systems.

This may require changes in local by-laws, as local regulations will, to a great extent, dictate the final choice. On the other hand, decentralised stormwater drainage only offers a solution for temporary retention, as the water will ultimately need to be transported to sewer systems. In some cases, the maintenance costs will be higher, but decentralised systems help to attain benefits like improved human well-being, absorption of air pollution and moisture retention, thus lowering ambient temperature and attenuating the urban heat island affect, ultimately contributing to the greening of cities.

Decentralised systems can also be used for the treatment of runoff from highways. Experience accumulates with the implementation and exploitation of decentralised sanitation and urban drainage. Nonetheless, some barriers have to be overcome, such as social perceptions and difficulties associated with retrofitting.

An additional challenge is the need to manage wastewater at different scales (from buildings to the municipal level, to even larger levels). These barriers can be overcome by a combination of information campaigns, a whole-of-government approach to urban water management (including policies, laws and regulation), business models for water utilities and land development that factor in externalities related to wastewater management, and a long-term vision of the challenges in the water sector and the opportunities for urban development.

Evolution of treatment technologies

Significant advances have been made in treatment technologies, since the original development of aerated systems (e.g. activated sludge and trickling filters) during the 1920s. The selection of treatment systems has been driven by the prevailing economic situation or by other factors like global warming, water scarcity, environmental quality issues and/or land use planning. In the rapidly urbanising centres worldwide, the prevention of the discharging of carbonaceous material was the priority in order to protect receiving waters being starved of oxygen.

The oxygen demand was ‘satisfied’ by using large amounts of energy to encourage the growth of microbial biomass (sludge), which was separated from the system and used in agriculture or dumped at sea. Later developments saw extended aeration systems to reduce the final amount of biomass for disposal, as this was responsible for a large proportion of the treatment costs. During the oil crisis in the 1970s, anaerobic digestion became the preferred method to treat wastewater and sludge, on account of the reduced amount of energy available.

The 1980s and 1990s saw an increased interest in nutrient removal, mainly in the developed world, as nutrient discharge had led to the eutrophication of water bodies in many regions of the world. During the same period, significant advances were made in the use of more natural treatment systems, such as waste stabilisation ponds and reed bed systems. These types of systems offer efficient reduction in pathogens with low capital and operational costs. Indeed, even in developed economies, they find a use in small-community treatment systems.

The most recent trends have seen treatment systems that address the reduction of GHG emissions. In parallel, much research was undertaken, particularly in the developing regions of the world, on systems that focused on reducing the bacteriological hazards.

Sewer mining and component separation

Active direct use of wastewater and the nutrients it contains has often been driven by necessity, but its use for recreation or other purposes has been documented in many developed regions. New technologies are emerging that allow for the upgrading of wastewater treatment plants to ‘factories’ in which the incoming materials are deconstructed to units such as ammonia, carbon dioxide and clean minerals.

This is followed by a highly intensive and efficient microbial re-synthesis process where the used nitrogen is harvested as microbial protein (at efficiencies close to 100%), which can be used for animal feed and food purposes. Another new approach has been proposed in which the used water is subjected to a procedure that allows the uptake of its organics and inorganics materials into fish biomass. The fish are harvested and processed to become a source of feed or food. The remaining water can be used for irrigation or discharged.

Indeed, the organics and inorganics present in the incoming used water are removed to a large extent in the form of the harvested fish. The key features of both of these concepts for treating wastewater is that they do not follow the route of destroying the nutritive value which is present in the used water.

On the contrary, they add a form of renewable energy to allow aerobic microbes to upgrade the nutrients to microbial cells growing in flocs, and they harvest the latter by fish grazing on them. In the latter case, biomass is then processed to become of further use as feed or food. Concerning the isolation and separation of useful wastewater components, it is likely that urine collection and use will become an increasingly important component of ecological wastewater management, as it contains 88% of the nitrogen and 66% of the phosphorus found in human waste.

 

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