a thirsty land

Luis Cáceres Villanueva
describes how wastewater can be used to green cities, even in difficult circumstances, and outlines how to treat and manage it

North Chile is a virtually rainless land. It contains six cities spread out in the Atacama desert, one of the driest regions of the world. Water supplies for urban areas are transported from distant wells or rivers close to the Andes mountains, where precipitation takes place. But a large fraction of the known supplies are affected by high concentrations of dissolved solids including boron and arsenic, which are present as soluble compounds in volcanic sediments, minerals and soils along the mountain range. This situation requires careful selection of water sources and water treatment methods to take out arsenic. Only about half of the water is suitable for direct human consumption: the rest is treated to remove its high arsenic content.

Municipalities, institutions and private owners face substantial challenges in providing the green areas and gardens required by urban dwellers. Greening initiatives are hampered by the high cost of water and by high salt and boron concentrations in the water and the soil, which severely affect the growth of sensitive ornamental plant species. A large fraction of the urban land set aside for recreation has ended up as empty fields or degraded parks. Universities, municipalities and private organizations launched a joint project to promote the development of appropriate green areas and gardens. It studied the feasibility of managing the use of available non-potable water through proper treatment and irrigation procedures. And it conducted field studies to select and evaluate native and introduced plants that could tolerate the salt and be used for ornamental purposes.

Reusing treated wastewater to irrigate green areas and vegetables was deemed to be most relevant. But its high salt content limits the range of acceptable crops to those resistant to it. Research established that it was convenient to use native plants as ornamental species because of their high resistance both to high salt concentrations in the soil and to water shortage.

Surprisingly, however, these species are not known to the local people – so it is necessary to promote their use through communication and training. Using native plants and available marginal-quality water – such as saline groundwater, and treated municipal and industrial wastewater – in this way are low-cost options for increasing green areas and gardens to develop a distinctive landscape for northern Chile.

Worldwide consensus
There is a general worldwide consensus on the benefits of wastewater recycling. But the design of any community system must bear in mind local factors affecting the process of treatment and reuse. It has not been possible, in many urban areas, to solve all the problems associated with where sewage ends up, because of economic difficulties, lack of adequate legislation, lack of interest in the community or lack of an organizational structure. The basic condition for implementing a wastewater treatment system is the existence – or establishment – of an efficient sewerage system to transport the wastewater to a treatment site. Another key factor is knowing the level and fluctuations in concentrations of chemicals that are hard or impossible to remove by conventional methods.

The choice of a wastewater treatment system is closely linked to the specific reuse intended – such as irrigating green areas or industrial use – and to the participation of interested sectors in generating ideas and projects. It can be difficult to finance any large-scale treatment project. So it is often appropriate to recommend localized projects – such as parks, gardens and orchards with their own system of sewage treatment – around populated areas, schools, hospitals, buildings and companies.
Using native plants and available marginal-quality water are low-cost options for increasing green areas and gardens
Natural systems, like stabilization reservoirs, are well developed, simple and inexpensive – and can be adapted to rural areas where land is cheap. Yet there is a marked preference for conventional processing of activated sewage sludge. This is apparently the result of publicity by manufacturers of modular plants. Any system of sewage treatment must take into account economic, social and environmental aspects – some of which are addressed by specific regulations – including bacteriological parameters, and concentrations of organic matter, heavy metals and sulphate. These all have foreseeable environmental impacts including eutrophication, which affects aquatic species; the emission of offensive smells from treatment plants operating under bad conditions; and the intrusion of salts caused by badly maintained drainage networks. These make it difficult to reuse water on sensitive plants in agriculture, lead to the loss of large quantities of scarce water in desert areas, and make it likely that diseases will spread through contact with sewage in bathing areas in rivers, lakes and on the coast.

Choosing instruments
High rates of disease are generally due to deficiencies in sewage collection and management systems. This is so in several Latin American countries, where programmes have begun to search for sources of contamination through sampling and measuring the load of coliform organisms and/or faecal organisms in rivers, coastal areas and other water bodies. Corrective measures include the control of treatment plants and/or dosing water bodies with chlorine. The formation of organo-chlorinated compounds resulting from this practice has not been considered an environmental problem requiring standardization, as it has in industrialized countries. Strong odours are common near treatment plants all over the world, and usually provoke complaints from the community. They are associated with high concentrations of sulphate and organic matter in the sewage. In most cases, the problem is solved by confining or eliminating anaerobic areas. The concentration of suspended solids generated during treatment or management frequently causes problems. Long accumulation of treated water in open tanks or in anaerobic conditions favours the growth of micro-algae or bacteria in suspension, obstructing irrigation accessories. Choosing instruments, including sand filters, which will avoid this problem is most important so as to avoid problems of obstruction: those that can be cleaned manually have proved to be the most convenient.

Managing the sewage sludge generated in the treatment plants is conditioned by its heavy metal content, mainly the result of releasing domestic cleaning products into the drain network. Disposing of highly contaminated sewage sludge causes constant controversy even in industrialized countries, due to the high costs of management and the regulations involved. In rural areas, the quality of the treated sewage sludge (free of bacterial activity, and chemically stabilized) is good enough for it to be used as a soil conditioner. Good sewage treatment requires the study of local conditions to establish strategies to eradicate or reduce adverse effects, and to harmonize selected technological treatment alternatives with environmental regulations

Professor Luis Cáceres Villanueva is Associate Professor of Chemical Engineering at the University of Antofagasta, Chile.


This issue:
Contents | Editorial K. Töpfer | Action for tomorrow | Turning words into action | One hand washes the other | People | Fragile resource | Realizing the dream | Washing away poverty | At a glance: Water and sanitation | Music makes magic – Angélique Kidjo | Targeting sanitation | In a city like Mumbai | Flowing from the bottom up | Books & products | Watering a thirsty land | Peace through parks | Reaching the unheard

Complementary issues:
Water, 1996
Freshwater, 1998
Biological Diversity, 2000
Freshwater, 2003