Design of modular wastewater treatment plants in small communities for in-situ reuse of the effluent

D. Hidalgo, R. Irusta, J. del Alamo and Y. NúñezEnvironment Division, FUNDACIÓN CARTIF, Parque Tecnológico de Boecillo, 47151 Boecillo, Valladolid, Spain (E.mail: dolhid@cartif.es)

Abstract

Water is an environmental, social and economic asset and as such needs to be managed with the objective of conserving a common patrimony in the interests of the community at large. Thus it is necessary and important to guarantee water availability over time by means of sustainable forms of management and treatment, which will allow countries to cope with present demands without jeopardizing environmental balance and the needs of future generations.

The main objective of wastewater treatment is to allow human and industrial effluents to be disposed of or reuse without danger to human health or damage to the natural environment. Pollution from poorly functioning or inadequate systems is a major cause of water quality problems for small communities. This problem is especially important when recycling.

Recycled water can satisfy most water demands, as long as it is adequately treated to ensure water quality appropriate for the use. In addition to providing a locally-controlled water supply, water recycling provides tremendous environmental benefits. By providing an additional source of water, water recycling can help to decrease the diversion of water from sensitive ecosystems. Other benefits include decreasing wastewater discharges and reducing and preventing pollution.

As for any water source that is not properly treated, health problems could arise from drinking, being exposed or consuming products irrigated with recycled water if it contains disease-causing organisms or other pollutants. This is the reason why every drop of wastewater to be reuse has to undergo previously a complete and appropriate treatment depending on the final use (Figure 1).

Figure 1. Treated municipal wastewater reuse in a small community

Wastewater, at different level of treatment (raw, secondary or tertiary) is used in small communities alone or mixed with fresh water, mostly on forage and cereals but also sometimes on fruit trees and even vegetables, depending on national legislation and its enforcement. In several cases the

wastewater is not properly treated due to the fact that the construction cost of efficient treatment systems is very high, especially for small and medium size communities.

If a community recognizes the need for improving wastewater treatment there are several options they can consider. Traditionally, the answer has been to rely on large projects covering a big area, although this is not always the most cost-effective solution. Centralised water supply and sewerage systems with up to 85% of cost related to piping and pumping seems inappropriate for different countries, developing areas and rural places. Conventional sewerage is expensive and water intensive and therefore its application for small communities cannot be justified. Within a county or city, there is likely to be a wide variety of wastewater treatment needs and the use of smaller community systems is a way to accommodate these differences.

Accelerated extension of wastewater management services to small communities is essential to address serious concerns over water scarcity, pollution and protection of public health. Following FAO recommendations, wastewater services must be developed to deliver the following benefits:- Protect public health and well being of the communities.- Meet the increasing demand for convenience.- Protect the scarce water resources and the household and community environment from pollution.- Alleviate the pressure on the scarce freshwater resources by maximizing reuse opportunities.

Of course many alternative solutions have been developed with the scientific and technological progress during the last years. However, the selection of the appropriate treatment technique that is tailored to the needs of each community means in several cases the involvement of qualified specialists. Moreover, in several cases the outflow of the wastewater treatment systems does not have a standard quality either because standard operating procedures are not followed, or because there is no qualified personnel able to overcome usual problems and to control/monitor the whole treatment procedure. On the other hand, in traditional static (without any control action) wastewater systems, the performance of the system cannot be adapted to the time varying conditions after the system is taken into operation. The limited efficiency in reducing flooding, environmental pollution and health risks is very often caused by the lack of flexibility in the operation of the static urban wastewater systems under dynamic loadings. Even if the system is correctly designed, there is no guarantee that the elements are at capacity simultaneously during each single event.

Until recently, little attention has been paid to the design and operation of small wastewater treatment plants, despite the vast number of such plants all over the world. There is a huge variation in the quantity and quality of the influent that these plants must treat, and in many cases, their continuous supervision is not feasible. Furthermore, it is essential to consider local circumstances, what make their design far from simple.

Small wastewater treatment plants (i.e. those covering the range 500-2000 population equivalent, PE) contribute in a huge manner to solving global pollution problems. However, their role is far from insignificant and locally they are often crucial in protecting watercourses and drinking-water sources that are of major importance. Small rural communities, factories, tourist and educational establishments and campsites can be served by this kind of facilities. Many communities could avoid costly construction projects through improved management skills, appropriate technology, and suitable wastewater treatment system operation and maintenance.

This paper covers the design of an automated and modular wastewater treatment plant able to be adapted to the treatment of a wide range of effluents (urban, agricultural or, even, biodegradable industrial effluents) over a broad range of technologies by using a low cost, prefabricated and modular production technology, that is easy to install and can be adapted to different conditions.

The whole treatment is composed of three stages. A first stage, pre-treatment, which depends on the wastewater characteristics; a modified Ludzack-Ettinger biological treatment, which is carried out in a set of prefabricated anoxic and aerobic modules and a secondary settler; and finally, a sludge treatment stage (Figure 2).

Figure 2. Modular treatment plant

The dimensional module features, 3,2 x 3 x 6 m3 vessels, make its transport easy and allow its implementation for the treatment of different types of effluents by joining several units. The anoxic and aerobic modules can be assembled in series or parallel. The required number of modules and their distribution will depend on the flow and charge in the inlet water, (very changeable in tourist areas), and the localisation characteristics (area where the plant will be settled). The modules and other parts of the plant can be made of different materials, stainless steel, carbon-steel, polyester, etc. Moreover, its building is faster than other plants because of civil works have been minimised.

The plant is highly automated which allows operators to undertake more light maintenance work. The intuitive and friendly control system is one of the largest advantages of the plant. The control is made by means of a personal computer and the software Scada. This software shows, in different screens, the plant performance and acts as a human-machine interface.

Summarizing, the main advantages of the system developed are:

- Modular design and construction to treat effluent from small communities (urban or industrial) of 500-200 PE (taking into account that PE does not necessarily reflect the actual population of a community), based on the choice of efficient and affordable wastewater treatment technologies which can be placed close to the human settlements without causing nuisance to the community.- Mechanically simple and robust - Low capital and maintenance costs with minimal operator attendance - Improve on-site wastewater treatment- Reduce wastewater transportation necessities- Maximize on-line wastewater reuse opportunities