Since the spring of 2000, when the small agricultural community of Walkerton in Canada had found in its water supply the unwelcomed presence of E.coli which caused the death of seven citizens, consumers and Canadian governmental agencies have started to show greater interest and concern with regards to the quality of water supplied by municipal networks. This incident catalyzed the adoption of better integrated water resource management practices and shows the importance of considering water as part of a societal cycle: the discharge from one user can become the source to another, as shown in Figure 1. It is thus important to be conscious of the impact that one can have on the other links in the cycle.
Figure 1. Societal cycle of water utilization
To ensure that all citizens gain access to a sufficient quantity of quality water, considerable investments are needed for the construction and maintenance of water supply networks and wastewater treatment facilities. As expected a large portion of these investments is related to the processes and technology used to treat both water before it is supplied to the consumers, and wastewater before it is discharged back into the receiving environment.
Wastewater can be characterized in terms of physical, chemical and biological composition and these characteristics are closely linked to the following principal constituents of concern in wastewater treatment:
Suspended solids: commonly measured as total suspended solids or TSS;
Biodegradable organics: commonly measured in terms of biochemical oxygen demand (BOD) and of chemical oxygen demand (COD);
Pathogens: some pathogenic organisms present in the wastewater can transmit communicable diseases;
Nutrients: among these, nitrogen and phosphorus are of particular importance;
Priority pollutants: these pollutants can be known or suspected carcinogens, mutanogens or be highly toxic, for example;
Refractory pollutants: these compounds tend to resist conventional methods of treatment and can be pesticides, for example;
Heavy metals: mainly come from commercial and industrial activities;
Dissolved inorganics: sodium, calcium and sulphate are examples of these compounds which may come from domestic water treatment.
Pollutants found in wastewater can in turn be eliminated with physical, chemical and biological methods. Physical methods are those in which physical forces dominate whereas in the case of chemical methods, chemical products are added to complete treatment. In biological treatment, processes use microorganisms to eliminate the dissolved organic pollution in wastewater, as well as, in certain cases, nitrogen and phosphorus compounds. The following paragraphs present an overview of the processes associated with each of these three categories.
Some of the unit operations most commonly used for physical treatment are screening, mixing and flocculation, sedimentation (see Figure 2), flotation, aeration and filtration. These methods are mostly implemented in order to remove suspended solids or oil and grease, but they can also serve to provide oxygen for microorganisms in a biological process (ex.: in aeration) or to remove dissolved organic and inorganic matter (in membrane filtration, for example). Membrane filtration processes have become increasingly important because of the concerns associated with priority and refractory pollutants that may not be removed by conventional treatment processes and may end up in the water supply of other users. Membrane processes are also very promising when used in a seawater desalination context, especially in regions of the world that have scarce freshwater resources.
Figure 2. Example of a sedimentation unit: a clarifier is used to settle biological solids in an activated sludge process.
The principal unit operations used in chemical treatment are coagulation, precipitation, disinfection, oxidation and ion exchange. Coagulation and precipitation can be used to enhance the physical removal of constituents. In the case of precipitation, TSS, BOD, phosphorus and heavy metals can be effectively removed. Disinfection is often associated with chlorine, chlorine compounds or ozone and refers to the partial destruction of disease-causing organisms. Oxidation and ion exchange can be used to remove organic compounds and ammonia nitrogen, for example.
Physico-chemical processes are mostly implemented in order to complete treatment by biological means. Physico-chemical treatment applications are nowadays rare to treat municipal wastewater and are mostly used in an industrial context. One major disadvantage linked to these types of processes is the handling, disposal or elimination of large quantities of generated sludge that will be responsible for a large portion of the operating costs of a treatment facility.
Finally, the principal objectives of biological treatment are (1) to transform biodegradable constituents into acceptable products, (2) to capture suspended solids into a biological floc or biofilm and (3) to transform or remove nutrients such as nitrogen or phosphorus. Biological processes can be divided into two main categories, suspended growth and attached growth processes. In suspended growth processes, the microorganisms are maintained in a liquid suspension by mixing while, in attached growth processes, they are attached to an inert packing system. The activated sludge process is the most common suspended growth process for municipal wastewater treatment. Microorganisms are supplied with adequate amounts of nutrients and oxygen in an aerated basin which also provides mixing. These organisms are then separated from the treated effluent in a clarifier (sedimentation unit) and recycled to the entrance of the basin for further treatment. This allows treatment facilities to operate with smaller hydraulic retention times and thus, need less space and capital costs. An activated sludge process can reach BOD and COD reductions of 85 to 98% and 60 to 85%, respectively. An example of an attached-growth process is the trickling filter in which wastewater is distributed over the top area of a unit containing packing material. In this case, the microorganisms grow as an attached biofilm.
Biological processes can also efficiently remove nitrogen from wastewater through the nitrification-denitrification cycle. Ammonia nitrogen (NH3) is first oxidized into nitrate (NO3) by two bacteria genera (Nitrosomonas and Nitrobacter) and then transformed, in absence of oxygen, in nitrogen gas (N2) which is inert and readily removed from wastewater. A wide range of bacteria are responsible for the latter reaction. Other types of bacteria, the so-called phosphorus accumulating bacteria, can also effectively store and consume phosphorus, thus removing it from wastewater.
Figure 3. Example of a biological treatment process, in this case, the aeration basin of an activated sludge process.