Wastewater treatment

= (7.1)

where the ionic concentrations of sodium (Na), calcium (Ca) and magnesium (Mg) are expressed in me/l. If significant precipitation or dissolution of calcium due to the effect of carbon dioxide (CO2), bicarbonate (HCO3-) and total salinity of the water (ECw) is suspected, the adjusted sodium adsorption ratio (SARadj) can be used as reported by Ayers and Westcot (1985).

ƒ Toxic ions. When at concentrations above threshold values, they can cause plant toxicity problems, which affect growth and yield of crops. The degree of damage depends on the crop, its stage of growth, and the concentration of the ions. The most common phytotoxic ions in municipal sewage and treated effluents are boron (B), chloride (C1), and sodium (Na).

ƒ Trace elements. Attention should be paid to trace elements in sewage effluents if industrial wastewater is included. The main ones are Aluminium (Al), Beryllium (Be), Cobalt (Co), Fluoride (F), Iron (Fe), Lithium (Li), Manganese (Mn), Molybdenum (Mo), Selenium (Se), Tin (Sn), Titanium (Ti), Tungsten (W) and Vanadium (V).

ƒ Heavy metals, a special group of trace elements, which have been shown to cause health hazards when taken up by plants: Arsenic (As), Cadmium (Cd), Chromium (Cr), Copper (Cu), Lead (Pb), Mercury (Hg) and Zinc (Zn).

ƒ pH. The normal pH range for irrigation water is from 6.5 to 8.4. pH values outside this range indicate water is abnormal in quality.

7.2.3. Wastewater treatment

Wastewater treatment aims at safe disposal of human and industrial effluents, without danger to human health or damage to the natural environment. Irrigation with wastewater is

both disposal and utilisation. Some degree of treatment needs to be provided to raw municipal wastewater before it can be used for agricultural and landscape irrigation, aquaculture or other uses. The required quality of effluent will depend on the proposed water uses, crops to be irrigated, soil conditions and the irrigation system. In aquaculture, more reliance will have to be placed on control through wastewater treatment.

The most appropriate wastewater treatment for agricultural uses is that which will produce an effluent meeting the recommended microbiological and chemical quality guidelines both at low cost and with minimal operational and maintenance requirements (Arar, 1988). Adopting as low a level of treatment as possible while achieving the desired results is important, especially in developing countries. In practice, it may be better to design the reuse system to accept a low-grade of effluent rather than to rely on advanced treatment processes to produce a reclaimed effluent which continuously meets a stringent quality standard.

The design of wastewater treatment plants is usually based on the need to reduce organic and suspended solids loads to limit pollution of the environment. Pathogen removal has very rarely been considered an objective but, for reuse of effluents in agriculture, this must be of primary concern (Hillman, 1988). Treatment to remove wastewater constituents that may be toxic or harmful to crops, aquatic plants and fish is normally not economically feasible. However, the removal of toxic elements and pathogens that may affect human health needs to be considered.

The daily variations in flow from a municipal treatment plant make it generally not feasible to irrigate with effluent directly from the treatment plant. Some form of short-term storage of treated effluent is necessary to provide a relatively constant supply of reclaimed water for efficient irrigation, although additional benefits result from storage in reservoirs.

7.2.3.1. Conventional wastewater treatment

Conventional wastewater treatment consists of a combination of physical, chemical, and biological processes and operations to remove solids, organic matter and, sometimes, nutrients from wastewater. Different degrees of treatment are considered (Pescod, 1992):

(a) Preliminary treatment, where the objective is the removal of coarse solids and other large materials from the raw wastewater. Treatment operations include coarse screening, grit removal in most small treatment plants and, in some cases, comminution (trituration) to reduce the size of large particles so as to remove them in the form of sludge in subsequent treatment processes.

(b) Primary treatment. Its objective is the removal of settable organic and inorganic solids by sedimentation, and the removal of materials that float by skimming. Large fractions of the biochemical oxygen demand (BOD5), total suspended solids, oil and grease are then removed. Some organic nitrogen, organic phosphorous and heavy metals associated with those solids are also removed, but not colloidal and dissolved constituents. It may be considered sufficient treatment if the wastewater is to be used to irrigate crops that are not consumed by humans or to irrigate orchards, vineyards, and some processed food crops. However, to prevent potential nuisance conditions in reservoirs, which may affect nearby populations and workers, some form of secondary treatment may be required even in the case of non-food crop irrigation.

Primary sedimentation tanks or clarifiers are used. The sludge of settled solids is removed from the bottom of tanks and floating solids are swept across the tank surface by water jets or mechanical means. In large sewage treatment plants, primary sludge is commonly processed biologically by anaerobic digestion. Gas containing methane is then produced and can be used as an energy source. In small sewage treatment plants, sludge is processed by aerobic digestion, storage in sludge lagoons, land application, and others.

(c) Secondary treatment, where the objective is the further treatment of the primary effluent to remove the residual organics and suspended solids. In most cases, secondary treatment involves the removal of biodegradable dissolved and colloidal organic matter using aerobic biological treatment processes. Several aerobic biological processes are used for secondary treatment, differing primarily in the manner in which oxygen is supplied to the microorganisms and in the rate at which microorganisms metabolise the organic matter.

High-rate biological processes are characterised by relatively small reactor volumes and high concentrations of microorganisms compared with low rate processes. The microorganisms must be separated from the treated wastewater by sedimentation to produce clarified secondary effluent. The biological solids removed during secondary sedimentation are normally combined with primary sludge for sludge processing.

Common high-rate processes include the activated sludge processes, trickling filters or biofilters, oxidation ditches, and rotating biological contactors (RBC). A combination of two of these processes in series (e.g., biofilter followed by activated sludge) can be used to treat municipal wastewater containing a high concentration of organic material from industrial sources.

High-rate biological treatment processes remove nearly 85% of the BOD5 and suspended solids as well as some of the heavy metals. Activated sludge generally produces an effluent of slightly higher quality than biofilters or RBCs. When coupled with a disinfection step such as chlorination, these processes can provide substantial but not complete removal of bacteria and viruses. However, they remove very little of the phosphorous, nitrogen, non-biodegradable organics, or dissolved minerals.

(d) Tertiary and/or advanced treatment, which is employed when specific undesirable wastewater constituents cannot be removed by secondary treatment. This may be the case for nitrogen, phosphorous, additional suspended solids, refractory organics, heavy metals and dissolved solids.

Where the risk of public exposure to the treated water is high, the intent of the advanced treatment is to minimise the probability of human exposure to enteric viruses and other pathogens. Because effective disinfection is believed to be inhibited by suspended and colloidal solids in the water, these solids must be removed by advanced treatment before the disinfection step. Therefore, the sequence of treatment often is secondary treatment followed by chemical coagulation, sedimentation, filtration, and disinfection. This level of treatment is assumed to produce an effluent free from detectable viruses.

(e) Disinfection. Disinfection normally involves the injection of a chlorine solution (5 to 15 mg/l) at the head end of a chlorine contact basin. Ozone and ultra violet irradiation can also be used to meet advanced wastewater treatment requirements. A chlorine contact time as long as 120 minutes is sometimes required. In Near East countries

adopting tertiary treatment, the tendency has been to introduce pre-chlorination before rapid-gravity sand filtration and post-chlorination afterwards. A final ozonation treatment after this sequence has seldom been considered (Al-Nakshabandi et al., 1997).

7.2.3.2. Natural biological treatment systems

Natural low-rate biological treatment systems for the treatment of municipal sewage tend to be less costly and sophisticated in operation and maintenance than the high-rate biological processes mentioned above. They may be more effective in removing pathogens if properly designed and not overloaded. Natural biological treatment systems consist of (Pescod, 1992):

(a) Stabilisation ponds. Stabilisation ponds are the preferred wastewater treatment process for effluent use in agriculture in developing countries. These systems are designed to achieve different forms of treatment in a series of three stages. The number of stages used will depend on the organic strength of the input waste and the effluent quality objectives. Strong wastewaters, having BOD5 > 300 mg/l, are introduced into first-stage anaerobic ponds, which achieve a high rate of removal.

Where anaerobic ponds are environmentally unacceptable (mainly because of odour and flies) or are not required (e.g. for weaker wastes i.e. with low BOD5), wastewater is discharged directly into primary facultative ponds. Effluent from first-stage anaerobic ponds will overflow into secondary facultative ponds which comprise the second-stage of biological treatment. Maturation ponds to provide tertiary treatment are introduced following the primary or secondary facultative ponds, if further pathogen reduction is necessary.

Solids in the influent to a facultative pond and excess biomass produced in the pond will settle out forming a sludge layer at the bottom. The benthic layer will be anaerobic and, as a result of anaerobic breakdown of organics, will release soluble organic products to the water column above. Organic matter dissolved or suspended in that water is metabolised by heterotrophic bacteria, with uptake of oxygen, as in conventional aerobic biological treatment processes. Unlike in conventional processes, the dissolved oxygen utilised by the bacteria in facultative ponds is replaced through photosynthetic oxygen produced by microalgae rather than by aeration equipment. High temperature and sunlight create conditions which encourage algae to utilise the carbon dioxide (CO2) released by bacteria in breaking down the organic components of the wastewater and to take up nutrients, mainly nitrogen and phosphorous. This contributes to the overall removal of BOD5 in facultative ponds. However, the organic loading must be strictly limited otherwise not enough oxygen will be produced. Wind is important to the satisfactory operation of facultative ponds for mixing the contents and to prevent thermal stratification that would cause anaerobiosis and subsequent failure of the processes.

The effluent from facultative ponds normally contains at least 50 mg/l BOD5. If lower BOD5 concentration is required it will be necessary to use maturation ponds. A more important function of maturation ponds, however, is the removal of excreted pathogens. Longer retention in anaerobic and facultative pond systems will make them more efficient than conventional treatment processes in removing pathogens.

Effluents from a facultative pond treating municipal sewage generally require further

treatment. Maturation ponds should then be designed to achieve a given reduction of faecal coliforms (FC). Protozoan cysts and helminth eggs are removed by sedimentation in stabilisation ponds. A series of ponds with overall retention of 20 days or more will produce an effluent totally free of cysts and ova (Feachem et al., 1983, cited by Pescod, 1992). Pathogen die-off, is linked to algal activity. The coliform and faecal coliform die-off coefficients vary with retention time, water temperature, organic loading, total BOD5 concentration, pH and pond depth (Saqqar, 1988, cited by Pescod, 1992).

(b) Overland treatment of wastewater. In overland flow treatment, the effluent is distributed over gently sloping grassland on fairly impermeable soils. Ideally, the wastewater moves evenly down the slope to collecting ditches at the bottom edge of the area. Suspended and colloidal organic materials are then removed by sedimentation and filtration through the surface grass and organic layers. Overland flow systems also remove pathogens from sewage effluent at levels comparable with conventional secondary treatment systems, without chlorination. This form of land treatment requires intermittent applications of effluent (usually treated) alternating with resting of the land, to allow the soil to react with the sediments and for grass cutting. Basic site characteristics and design features for overland flow treatment have been suggested (EPA, 1977, cited by Pescod, 1992). The impact on groundwater should be considered in the case of highly permeable soils. The application rate for wastewaters will depend principally on the type of soil, the quality of wastewater effluent and the physical and biochemical activity in the near-surface environment (Middlebrooks et al., 1982, cited by Pescod, 1992).

The cover crop is an important component of the overland flow system since it should prevent soil erosion, provide nutrient uptake and serve as a fixed-film medium for biological treatment. Crops best suited to overland flow treatment are grasses with a long growing season, high moisture tolerance and extensive rooting. Reed canary grass, rye grass and tall fescue are among the suitable species.

(c) Macrophyte treatment. This occurs in maturation ponds that incorporate floating, submerged or emergent aquatic species (macrophytes). They can be used for upgrading effluents form stabilisation ponds, thus acting as maturation ponds.

Macrophytes take up large amounts of inorganic nutrients (N and P) and heavy metals (Cd, Cu, Hg and Zn).

Among the floating macrophytes, having large root systems and very efficient nutrient extraction, are the water hyacinth, Eichornia crassipes, able to double in mass about every 6 days, and the coontail, Ceratophyllum demersum. The aquatic vascular plants also serve as living substrates for microorganisms that remove BOD and nitrogen, and achieve reductions in phosphorus, heavy metals and some organics through plant uptake. The basic function of the macrophytes in the latter mechanism is to assimilate, concentrate and store contaminants on a short-term basis. Subsequent harvest of the plant biomass results in permanent removal of stored contaminants from the pond treatment system. Fly and mosquito breeding are a problem in floating macrophyte ponds. This nuisance can be partially controlled by introducing into the ponds fish species such as Gambusia and Peocelia that eat these larvae. Pathogen die-off is poor in macrophyte ponds as a result of light shading and the lower dissolved oxygen and pH.

Natural and artificial wetlands and marshes having emergent macrophytes can also be used to treat raw sewage and partially-treated effluents. The main species of emergent macrophytes (reeds) are Phragmites communis and Scirpus lacstris. These macrophytes not only take up the inorganic nutrients but also create a favourable environment in the root zone for microorganisms, through pathways created by their highly developed root systems. BOD and nitrogen are then removed by bacterial activity. Aerobic treatment takes place in the rhizosphere, since oxygen passes to it from the atmosphere through leaves, stems, and roots, while an anoxic and anaerobic treatment takes place in the surrounding soil. Suspended solids in the sewage are aerobically composted in the above ground layer of decaying vegetation formed from dead leaves and stems. Nutrients and heavy metals are then removed by plant uptake.

The growth rate and pollutant assimilative capacity of emergent macrophytes such as Phragmites communis and Scirpus lacstris are limited by the culture system, wastewater loading rate, plant density, climate and management factors.

(d) Nutrient film technique. The nutrient film technique (NFT) is a modification of the hydroponics plant growth system, in which plants are grown directly on an impermeable surface to which a thin film of wastewater is continuously applied. Root production above the impermeable surface is high and the large root surface area traps and accumulates matter. Plant growth produces nutrient uptake, and provides shading for protection against the development of algae, while the large mass of roots and accumulated material serve as living filters.