Monitoring of Wastewater Quality: a Review

Abstract

Real-time monitoring of wastewater quality remains as unresolved problem to the waste water treatment industry. In order to comply with increasingly stringent environmental regulation, plant operators as well as industrial manufactures have expressed the the need of new standard and improved comparability of existing techniques. A review of currently available methods for monitoring global organic parameters (BOD,COD,PH,DO etc) is given. The study review both existing standard techniques and new innovative technologies with the focus on the sensors’ potential for on-line and real-time monitoring and control. Current developments of virtual sensors for the monitoring of wastewater organic load are presented and the interests and limitations of these techniques with respect to their application to the wastewater monitoring are discussed.

Key words: BOD; COD; DO;PH, waste water, virtual sensor.

Introduction

Water is the precious gift of nature to the humanbeing. The European Community decided in 1991 to oblige all EU Member States to equip with wastewater treatment plants all cities whose wastewater organic load is greater than 15 000 equivalent in habitants (to be implemented before 3 1 December 2000) and 2000 equivalent-inhabitants (to be implemented before 31 December 2005 ). The characterization of wastewater at the inlet and outlet of the treatment plants is an effective way to control the process efficiency and to verify the final quality of treated waters. Usually, wastewater quality is characterized both by global parameters like biological oxygen demand (BOD), chemical oxygen demand (COD), total organic carbon (TOC) or total suspended solids (TSS), and by nitrogen and phosphorus compounds. All values must be lower than the maximum permissible values, depending on specific regulations.

These dispositions are of great importance but unfortunately the great importance but unfortunately the monitoring procedures presently performed are not very satisfactory because they involve sampling, storage and laboratory analysis - a succession of sample handlings which considerably enhances the risk of errors. There is now an increasing need to limit sample handlings and to develop fast and accurate devices enabling a range of parameters to be monitored by direct field measurements. The aim of this study was to review both existing standard techniques and new innovative technologies with the focus on the sensors’ potential for on-line and real-time monitoring and control.

Review of Literature

The world is faced with problems related to the management of wastewater. This is due to extensive industrialization, increasing population density and high urbanized societies (EPA, 1993; McCasland et al. , 2008). The effluents generated from domestic and industrial activities constitute the major sources of the natural water pollution load. This is a great burden in terms of wastewater management and can consequently lead to a point-source pollution problem, which not only increases treatment cost considerably, but also introduces a wide range of chemical pollutants and microbial contaminants to water sources (EPA, 1993, 1996; Eikelboom and Draaijer, 1999; Amir et al. , 2004). The prevention of pollution of water sources and protection of public health by safeguarding water supplies against the spread of diseases, are the two fundamental reasons for treating wastewater. This is accomplished by removing substances that have a high demand for oxygen from the system through the metabolic reactions of micro organisms, the separation and settling of solids to create an acceptable quality of wastewater effluents, and the collection and recycling of microorganisms back into the system, or removal of excess microorganisms from the system (Abraham et al. , 1997).

In municipal wastewater treatment systems, the common water quality variables of concern are biological oxygen demand (BOD), chemical oxygen demand (COD), dissolved oxygen (DO), suspended solids, nitrate, nitrite and ammonia nitrogen, phosphate, salinity and a range of other nutrients and trace metals (DeCico, 1979; Brooks, 1996). The presence of high concentrations of these pollutants above the critical values stipulated by national and international regulatory bodies is considered unacceptable in receiving water bodies. This is because, apart from causing a major drawback in wastewater treatment systems, they also lead to eutrophication and various health impacts in humans and animals (EPA, 2000; CDC, 2002; Runion, 2008). In recent years, the reuse of treated effluent that is normally discharged to the environment from municipal wastewater treatment plants is receiving an increasing attention as a reliable water resource. In many countries, wastewater treatment for reuse is an important dimension of water resources planning and implementation. This is aimed at releasing high quality water supplies for potable use. Some countries, such as Jordan and Saudi Arabia have national policies to reuse all treated wastewater effluents, thus have made considerable progress towards this end.

In China, sewage use in agriculture has developed rapidly several decades ago and millions of hectares are irrigated with sewage effluent. The general acceptance is that wastewater use in agriculture is justified on agronomic and economic grounds, although care must be taken to minimize adverse health and environmental impacts (FAO, 1992; Metcalf and Eddy, 2003; Rietveld et al. , 2009; Sowers, 2009). [2004] have studied the industrial wastewater and ground water, and pollution problem in ground water. V. Singh and C. P. S. Chandel [2006] have analyzed the wastewater of Jaipur City, which is used for agricultural purpose Furthermore, wastewater reuse is increasingly becoming important for supplementing drinking water needs in some countries around the world. The option of reuse of wastewater is becoming necessary and possible as a result of increased climate change, thus leading to droughts and water scarcity, and the fact that wastewater effluent discharge regulations have become stricter leading to a better water quality (Rietveld et al. , 2009).

Characteristic Sources

1. Turbidity Erosion from upland, riparian, stream bank, and stream channel areas;
2. Color Domestic and industrial wastes, natural decay of organic materials 3. Odour Decomposing wastewater, industrial wastes.
3. Temperature Domestic and industrial wastes
4. PH Domestic, commercial, and industrial wastes
5. Chlorides Domestic wastes, domestic water supply, groundwater infiltration
6. Nitrogen Domestic and agricultural wastes

Principles and Classification of Existing Techniques

In addition to traditional laboratory-based analytical techniques used in the water industry, recent years have seen the development of a range of innovative monitoring equipment. Although only a small number of such product has yet reached the market or has been accepted, there is already a great diversity of techniques and technologies available, both commercially and in research laboratories, which are reported in the literature. As a consequence, different schemes have been used in an attempt to classify existing sensors and analysers according to their respective properties. (Lynggaard-Jensen1999) listed eight different sensor/analyser properties.

1. Placement of sensor In-situ, at-line, in-line; on-line, off-line
2. Principle of sampling External sampling, no external sampling
3. Principle of filtration Filtration, no filtration
4. Principle of sample treatment Continuous, batch
5. Principle of measurement Photometric, colorimetric, enzymatic, titrimetric, etc 6. No. measurants Single parameter, multi parameter
6. Need for supplies Consumables, no consumables
7. Service intervals Long, medium or short interval.

Relevant sensor properties (after Lynggaard-Jensen A 1999) which should be taken into consideration before their introduction into wastewater systems (ie for monitoring or process control). Indeed key features such as the cost of ownership, ease of use, placement of the sensors, as well the response time, will influence the consumer’s choice. Other technical aspects such as the principle of measurement, reliability, accuracy and detection limits will also dictate whether or not the technology will be accepted and promoted as astandard (or alternative) method by the end user and relevant authorities. It is, therefore, evident that both the performance characteristics (range, linearity, accuracy response time, limit of detection, etc. ) and the intrinsic properties of the sensors (single or multiparameter, need for external sampling and filtration, intrusive/non-intrusive) are of major importance when looking at existing and new methodologies for wastewater systems.

Assessment of Wastewater Quality

For the assessment of waste water pollution status of the water bodies, the following water quality parameters were analyzed.

1. Turbidity; Sewage is normally turbid, resembling dirty dish water or wastewater from baths having other floating matter like fecel matter, pieces of paper, cigarette-ends, match-sticks, greases, vegetable debris, fruit skins, soaps, etc. , The turbidity increases as sewage becomes stronger. The degree of turbidity can be measured and tested by turbidity rods or by turbidimeters, as is done for testing raw water supplies.
2. Colour; The colour of sewage can normally be detected by the naked eye, and it indicates freshness of sewage. If its colour is yellowish, grey, or light brown, it indicates fresh sewage. However, if the colour is black or dark brown, it indicates stale and septic sewage. Other colour, may also be formed due to the presence of some specific industrial wastes.
3. Odour; Fresh sewage is practically odourless. But however, in 3 to 4 hours, it becomes stale with all oxygen present in sewage being practically exhausted. It then starts omitting offensive odours, especially that of hydrogen sulphide gas, which is formed due to decomposition of sewage. The odour of water or wastewater can be measured by a term called the Threshold odour number (TON), which represents the extent of dilution required to just make the sample free of odour. The minimum odour of the sample that can be detected after successive dilutions with odourless medium, is, thus, known as the threshold odour. The Threshold odour number (TON) can be calculated by the equation: TON=(Vs+Vd)/VsWhere TON=Threshold Odour Number=Volume of the sewage=Volume of distilled or odourlesswater added to just make thesewage sample loss its colour.
4. Temperature; The temperature has an effect on the biological activity of bacteria present in sewage, and it also affects the solubility of gases in sewage. In addition, temperature also affects the viscosity of sewage, which, in turn, affects the sedimentation process in its treatment. The normal temperature of sewage is generally slightly higher than the temperature of water, because of additional heat added during the utilisation of water. The average temperature of sewage in India is 20ºC, which is near about the ideal temperature for the biological activities.

However, when the temperature is more, the dissolved oxygen content (D. O. ) of sewage gets reduced. 3. 5 The pH: The hydrogen ion concentration expressed as pH, is a valuable parameter in the operation of biological units. The pH of the fresh sewage is slightly more than the water supplied to the community. However, decomposition of organic matter may lower the pH, while the presence of industrial wastewater may produce extreme fluctuations. Water and wastewater can be classified as neutral, acidic or alkaline according to the following range: PH=7 neutral. PH>7 alkaline.