Wastewater: Problem and its Treatment | Ecology

After reading this article you will learn about:- 1. Introduction to Wastewater Treatment 2. Problem Statement of Wastewater Treatment 3. Treatment Technology 4. Treated Wastewater Reuse.

Introduction to Waste Water Treatment:

According to the World Bank, “the greatest challenge in the water and sanitation sector over few decades will be the implementation of low cost sewage treatment that will at the same time permit selective reuse of treated effluents for ag­ricultural and industrial purpose”.

It is crucial that sanitation systems have high levels of hygienic standards to prevent the spread of diseases. Other treatment goals include the recovery of nutrient and water resources for reuse in agricultural pro­duction and to reduce the overall user-demand for water resources.

In order to achieve ecological wastewater treat­ment, a closed-loop treatment system is recom­mended. Many present day systems are a “disposal-based linear system”. The traditional linear treat­ment systems must be transformed into the cycli­cal treatment to promote the conservation of wa­ter and nutrient resources.

Using organic waste nutrient cycles, from point-of-generation to point- of-production, closes the resource loop and pro­vides an approach for the management of valu­able wastewater resources. Failing to recover or­ganic wastewater from urban areas means a huge loss of life-supporting resources than instead of being used in agricultural for food production, Fill Rivers with polluted water.

The development of ecological wastewater management strategies will contribute to the reduction of pathogens in sur­face and groundwater to improve public health. “The goal of ecological engineering is to attain high environmental quality, high yields in food and fiber, low consumption, good quality, high effi­ciency production and full utilisation of wastes”.

In the growing number of conflicts between agricultural and domestic use of scarce water re­sources, an increased use of treated wastewater for irrigation purpose is vital. Wastewater is com­posed of over 99% water. In a developing urban society, the wastewater generation is usually ap­proximately 30-70 m³ per person per year.

In a city of one million people, the wastewater gener­ated would be sufficient to irrigate approximately 1500-3500 hectare. Innovative and appropriate technologies can contribute to urban wastewater treatment and reuse.

Based on extensive success­ful experience in Canada and elsewhere on cost effective and environmentally sound practices of sludge application on agricultural land, there is tre­mendous potential for the safe disposal of sew­age sludge on agricultural land.

Problem Statement of Wastewater Treatment:

Problems concerning water sanitation stem from the rise in urban migration and the practice of discharging untreated wastewater. The uncon­trolled growth in urban areas has made planning and expansion of water and sewage systems very difficult and expensive to carry out. In addition, many of those moving to the city have low in­comes, making it difficult to pay for any water sys­tem upgrades.

In developing countries, 300 mil­lion urban residents have no access to sanitation and it is mainly low-income urban dwellers who are affected by lack of sanitation infrastructure. Approximately two-thirds of the population in the developing world has no hygienic means of dis­posing excreta and an even greater number lack adequate means of disposing of total wastewater.

It is a common practice to discharge untreated sewage directly into bodies of water or put onto agricultural land, causing significant health and economic risks. While the number of households with access to drinking water supply has increased (approximately eighty per cent in Latin America and the Caribbean), the per cent connected to ur­ban sewage collection systems is only five per cent.

The effects of inadequate treatment can be detrimental to a community on economic, cultural and health-levels. The costs of poorly managed domestic waste are very high. In India, the 1994 plague epidemic resulted in a loss of tourism rev­enue estimated at $200 USD million; in Peru, a recent cholera epidemic resulted in an estimated loss amounting to three times the expenditure on water and sanitation for the entire country over the preceding 10 years; and in Shanghai, China, a recent major outbreak of Hepatitis A was attrib­uted to sewerage contamination.

Water contami­nated by human, chemical or industrial wastes can cause a number of diseases through ingestion or physical contact. Water-related diseases include dengue, filariasis, malaria, onchocerciasis, trypano­somiasis and yellow fever. Consequently, no other type of intervention has greater impact upon a country’s development and public health than the condition of clean drinking water and the appro­priate disposal of human waste.

The benefits of reusing treated wastes must also be measured against the cost of not doing so at both the economic and environmental level. The costs of implementing zero-discharge organic waste to agriculture recycling schemes may not be expensive. Full-scale implementation of urban or­ganic waste to agriculture systems could cost as little as $5-6 USD million for a city of 1 million people.

The problem with the current treatment tech­nologies is they lack sustainability. The conven­tional centralised system flushes pathogenic bacteria out of the residential area, using large amounts of water and often combines the domestic waste­water with rainwater, causing the flow of large volumes of pathogenic wastewater.

In fact, the conventional sanitary system transfers a concen­trated domestic health problem into a diffuse health problem for the entire settlement and/or region. In turn, the wastewater must be treated where the cost of treatment increases as the flow increases.

The abuse of water use for diluting hu­man excreta and transporting them out of the settlement is increasingly questioned and being considered unsustainable. The negative effects of centralised treatment are sum­marised in Table 19.1.

Another reason is that many treatment sys­tems in developing countries are not successful and therefore unsustainable are that they were sim­ply copied from western treatment systems with­out considering the appropriateness of the tech­nology for the culture, land, and climate.

Often local engineers educated in the western develop­ment programs supported the choice for the in­appropriate systems. Many of the implemented installations were abandoned due to the high cost of running the system and repairs.

On the other hand, conventional systems may even be technologically inadequate to handle the locally produced sewage. For example, in compari­son to the US and Europe, domestic wastewater in arid areas like the Middle East are up to five times more concentrated in the amount of oxygen demand per volume of sewage. This is extremely high and may cause a large amount of sludge production.

Appropriate Treatment Technology:

Based on experience from past mistakes in sew­age treatment technology, the definition of what is sustainable is clearer. Developers should base the selection of technology upon specific site con­ditions and financial resources of individual com­munities. Although site-specific properties must be taken into account, there are core parts of sus­tainable treatment that should be met in each case. The criteria for sustainable technology are summarised in Table 19.2.

One approach to sustainability is through de­centralisation of the wastewater management sys­tem. This system consists of several smaller units serving individual houses, clusters of houses or small communities. Black and gray water can be treated or reused separately from the hygienically, more dangerous excreta.

Non-centralised systems are more flexible and can adopt easily to the local conditions of the urban area as well as grow with the community as its population increases. This approach leads to treatment and reuse of water, nutrients, and by-products of the technology (i.e., energy, sludge, and mineralized nutrients) in the direct location of the settlement.

Communities must take great care when reus­ing wastewater both chemical substances and bio­logical pathogens threaten public health as well as accumulate in the food chain when used to irri­gate crops or in aquaculture. In most cases, Indus­trial pollution poses greater risk to public health than pathogenic organisms.

Therefore, more em­phasis is being placed on the need to separate do­mestic and industrial waste and to treat them indi­vidually to make recovery and reuse more sustain­able. The system must be able to isolate industrial toxins, pathogens, carbon, and nutrients.

1. Sustainable Treatment Types:

Now that the requirements for a sustainable waste­water treatment system have been presented, there are several options one can choose from in order to find the most appropriate technology for a par­ticular region. This paper will discuss sustainable. Wastewater treatment systems including la­goons/wetlands, UASB (anaerobic digesters), Hy­brid reactor, and SAT technologies.

2. Lagoons and Wetlands:

In wetland treatment, natural forces (chemical, physical, and solar) act together to purify the waste­water, thereby achieving wastewater treatment. A series of shallow ponds act as stabilisation lagoons, while water hyacinth or duckweed act to accumu­late heavy metals, and multiple forms of bacteria, plankton, and algae act to further purify the water.

Wetland treatment technology in developing coun­tries offers a comparative advantage over conven­tional, mechanised treatment systems because the level of self-sufficient; ecological balance, and eco­nomic viability is greater.

The system allows for to­tal resource recovery. Lagoon systems may be con­sidered a low-cost technology if sufficient, non-arable land is available. However, the availability of land is not generally the case in big cities. The de­mand of flat land is high for the expanding urban developments and agricultural purposes.

The decision to use wetlands must consider the climate. There are disadvantages to the system that in some locations may make it unsustainable. Some mechanical problems may include clogging with sprinkler and drip irrigation systems, particu­larly with oxidation pond effluent.

Biological growth (slime) in the sprinkler head, emitter ori­fice, or supply line cause plugging, as do heavy concentrations of algae and suspended solids. Other disadvantages are listed in Table 19.3.

3. Anaerobic Digestion:

Another treatment option available, if there is little access to land, is anaerobic digestion. Anaerobic bacteria degrade organic materials in the absence of oxygen and produce methane and carbon di­oxide. The- methane can be reused as an alterna­tive energy source (biogas).

Other benefits include a reduction of total bio-solids volume of up to 50-80% and a final waste sludge that is biologi­cally stable can serve as a rich humus for agricul­ture have low sludge production and low energy needs. Since nitrogen and phosphorus are not ef­fectively reduced in anaerobic technologies, this primary treatment approach works well with agri­culture or aquaculture.

However, they are not com­pletely effective at removing all pathogens, the wastewater needs a post treatment option to meet discharge standards, such as composting digested sludge, wetland systems, or stabilisation ponds (Table 19.4).

The UASB reactor essentially consists of a gas- solids separator (to retain the anaerobic sludge within the reactor), an influent distribution sys­tem, and effluent draw-off facilities, See Fig. 19.1 below for a schematic of UASB re­actor.

It is constructed with entrance pipes deliv­ering influent to the bottom of the unit and a gas solids separator at the top of the reactor to Sepa­rate the biogas from the liquid phase (water and sludge) overall, this prevents sludge washout.

The UASB system with a stabilisation pond for secondary treatment can cost $4 USD per person equivalent compared to $8 USD per person equiva­lent for activated sludge treatment. These costs would be for a system scale of 50,000 person equiva­lents if the land cost is less than $20 USD.

The hybrid reactor is an improved version of the UASB system and combines the merits of the up-flow sludge blanket and the fixed film reactors. The advantages include simplicity of design and operation; it also is- more economical than a fixed bed system.

4. Hybrid Reactor:

Wastewater treatment by the hybrid reactor sys­tem has become widespread as it provides advan­tages of both the suspended and attached growth phase at the same time. It may be used to treat some rate-limiting substrate, priority pollutants, volatile organic compounds etc. as well as for ni­trification.

This versatile nature of hybrid reactor demands for a detailed investigation on the mecha­nism, mode of operation, different applications and major configurations available. The present article is devoted to explore these issues with re­spect to previous background and successive de­velopment in this area.

Apart from the laboratory and pilot-scale study, some industrial applications have been overviewed to understand the perfor­mance of hybrid reactor in the concerned field. The approach of modelling for the hybrid reactor system is also demonstrated with the hypothetical data set. A comprehensive details about major hybrid processes is presented along with their sche­matic diagrams.

The review on hybrid process revealed that it would be economic for upgradation of existing activated sludge system, ensuring car­bonaceous oxidation and nitrification in a single reactor and treatment of slowly biodegradable substances also.

5. Soil Aquifer Treatment:

Soil aquifer treatment (SAT) is a geo-purification system where partially treated sewage effluent ar­tificially recharges the aquifers, and then withdrawn for future use. By recharging through unsaturated soil layers, the effluent achieves additional purifi­cation before it is mixed with the natural ground­water.

In water scarce areas, treated effluent be­comes a considerable resource for improved groundwater Sources. The Gaza Coastal Aquifer Management program includes treated effluents to strengthen the groundwater, in terms of both quantity and quality. With nitrogen reduction in the wastewater treatment plants, the recharged ef­fluent has a potential to reduce the concentration of nitrates in the aquifer.

In water scarce areas such as in the Middle East and parts of Southern Africa, wastewater has become a valuable resource that, after appropriate treatment, becomes a commer­cially realistic alternative for groundwater recharge, agriculture, and urban applications (Fig. 19.2).

SAT systems are inexpensive, efficient for pathogen removal, and operation is not highly tech­nical. Most of the cost associated with an SAT is for pumping the water from the recovery wells, which is usually $20-50 USD per m³.

In terms of reductions, SAT systems typically remove all BOD, TSS, and pathogenic organisms from the waste and tend to treat wastewater to a standard that would generally allow unrestricted irrigation. The biggest advantage of SAT is that it breaks the pipe-to-pipe connection of directly reusing treated wastewater from a treatment plant. This is positive attribute for those cultures where water reuse is taboo.

The pretreatment requirements for SAT vary depending on the purpose of groundwater recharge, sources of reclaimed water, recharge methods, and location. Some may only need primary treatment or treatment in a stabilisation pond. However; pre­treatment processes should be avoided if they leave, high algae concentrations in the recharge water. Algae can severely clog the soil of the infiltration basin.

While the water recovered from the SAT sys­tem has much better water quality than the influ­ent, it could still be lower quality than the native groundwater. Therefore, the system should be de­signed and managed to avoid intrusion into the na­tive groundwater and use only a portion of the aqui­fer.

The distance between infiltration basins and wells or drains should be as large as possible, usu­ally at least 45 to 106 m to allow for adequate soil-aquifer treatment. All the systems described allow for the reuse of treated wastewater in order to have a cyclic, sustainable system.

These treated wastewater pro­vide essential plant, nutrients (nitrogen, phospho­rus, and potassium) as well as trace nutrients. Phos­phorus is an especially important nutrient to re­cycle, as the phosphorus in chemical fertiliser comes from limited fossil sources.

The applica­tion of treated wastewater, as well as sludge, has considerable potential in a cyclical approach to crop applications, provided health risks and quality re­strictions are taken into consideration. Public health is the most critical issue regarding reclaimed wastewater.

Treated Wastewater Reuse:

Wastewater reuse must meet certain controls. First, wastewater treatment to reduce pathogen concen­trations must meet the WHO (1989) guidelines in Table 19.5. Second, crop restrictions must be speci­fied to prevent direct exposure to those consum­ing uncooked crops as well as defining application methods (irrigation) that reduce the contact of wastewater with edible crops.

Finally, control of human exposure is needed for workers, crop- handlers and final consumers.

It is well known that human waste is very much rich in nutrients (Table 19.6). But it field applica­tion need for societal support. Engineers and the local residents is necessary early on in the project, and if local participation is extensive, capital costs can ultimately be reduced.

According to the Inter- American Development Bank, “Citizen participa­tion, properly channeled, generates savings, mo­bilizes financial and human resources, promotes equity and makes a decisive contribution to the strengthening of society and the democratic sys­tem”.

There is a strong sense of ownership by mem­bers of the community in their projects. This pride in the new development helps to ensure the sustainability of the water supply and sanitation systems. Once the project is implemented, local participation contributes to the community’s con­fidence in the new technology and allows them to take on other challenges such as accessing finan­cial aid for other infrastructure projects.

On the governmental level, institutional strengthening is usually needed to assist small to medium-sized cities in dealing with new adminis­trative and financial management responsibilities. One program that has been developed to address the problems associated with decentralisation is RIADEL (Local Development Research and Ac­tion Network).

It is a network for sharing infor­mation about local community development in Latin America. It includes decentralisation and the training of social leaders and civil servants.

Case Studies and Current Research Activities:

There are several research and development projects on wastewater treatment, some have been successful and sustainable and some have not. The reasons for success or failure most often depend on the appropriateness of the implemented tech­nology. The following description is a perfect example of the in-appropnateness of adapting West­ern technology without making adjustments for the local environment.

In the 1970s, a foreign country donated a conventional activated sludge plant to the city of Amman, Jordan. Due to the arid climate, however, sewage in Jordan has ex­tremely high concentrations of organic matter.

This caused several problems in the plant such as: high-energy consumption for aeration, high vol­ume of sludge production, operational problems in the operational plant, and high consumption of polymers and clean water for drying the sludge after digestion.

Next, they implemented another unsustainable technology by constructing one of the world’s largest stabilisation ponds. Soon after the pond was installed, the plant was operating at loading rates double that of the design load caus­ing very poor effluent quality. Recently, another Western program installed off-gas treatment to prevent odor by placing surface aerators in the maturation ponds.

However, operation costs of the aerators were too high and the system stopped after two months. Not only was it expensive, but it also didn’t fix the odor problems since the odor­ous gases were coming off the anaerobic ponds and there was little improvement in effluent qual­ity.

One alternative treatment technology that would have supported the high COD quality of the effluent would have been anaerobic digestion. As explained previously, anaerobic digesters are generally low-tech, have low energy usage, and are less expensive to maintain.

The next case study, on the other hand, at­tempts to find a proper system for the country at a low cost to the community, and shows that in areas like the Middle East and Southern Africa where there is a shortage of water, groundwater recharge and agricultural/urban applications of treated effluent can be sustainable solutions.

In this case, Windhoek, Namibia is the location for a successful project implementing treated wastewa­ter reuse. Because the arid climate and water short­age were taken into account when determining the technology, the project incorporated SAT systems to recharge the groundwater and water demand management, based on IWRM (Integrated Waste­water and Recycling Management).

The required volume of water used to irrigate parks, sports fields, etc. has lowered since 1987, even though the population has doubled from approximately 105,000 to 202,000 over the same time. The artifi­cial recharge of aquifers was beneficial due to the lower evaporation, which allowed for water sup­ply during droughts. In addition, a feasibility study showed the system’s total investment cost would be recovered within five years.

Conclusion:

Thus it is essential to treat wastewater by cheap­est methods. The first was by decentralizing the treatment rather than installing expensive sewer systems that combine and increase the volume of the waste. The next involved choosing an appro­priate treatment technology for the community, where several types of proposal included lagoons/ wetlands, UASB (anaerobic digester), hybrid reac­tor, and SAT.

The common characteristic of all of the described types is that they encourage “zero-discharge” technology. This cyclical, rather than linear approach includes the reuse of the treated effluent for agricultural reuse. The reuse of the wastewater decreases the money spent on fertilisers and it is considered safe, since it has been treated for pathogens.

The urban areas of many developing countries are growing rapidly, ecological sanitation systems must be implemented that are sustainable and have the ability to adopt and grow with the community’s sanitation needs. In order to decide what the ap­propriate treatment system is, the developer must consider the area’s climate, topography, and socio­economic factors.

There are still plenty of needs in this area for research to improve or optimize the current methods of wastewater treatment. The re­sult of increased attention to this topic will improve the health, economic, and agricultural factors of a developing community.