(b) stimulation of algae growth and also a shift in the algal flora to the blue-green algae, leading to production of obnoxious blooms, floating scums or blankets of algae, etc. Most of the bloom-forming cyanophytes seem not to be utilized as food by invertebrates or zooplankton; thus predatory control becomes minimized. Decomposition of such algal masses in turn leads to oxygen depletion. Sewage discharge into waterways can lead to the spread of waterborne diseases, but the most important effect is that sewage increases biological productivity and this can in turn affect the diverse uses of the waterway.
Most municipalities face the problem of managing the ever-increasing volume of municipal solid waste (MSW) in an environmentally acceptable manner. While an integrated strategy using the “Four Rs” (Reduce, Reuse, Recycle, Recover) is regarded as the most efficient and cost-effective way of reducing the MSW requiring landfill, some environmental groups have been reluctant to endorse the use of all four Rs. An over-emphasis on recycling and firm opposition to recovery of energy by waste incineration has resulted in continued reliance on landfilling. Given the current landfill capacity crisis, incineration is likely to gain support as an integral part of an effective waste management strategy. Historically, public opposition to the sitting and operation of MSW processing and disposal plants has been based on concerns about emissions to the atmosphere. With the advent of more efficient operations and better air pollution control systems, the concern is shifting to disposal or use of the residues generated by -waste processing. For example, ash disposal is one of the major issues now limiting the acceptance of new MSW incinerator facilities.
Concerns about residue disposal are based on the potential for fugitive dust emissions and contamination of groundwater by trace metals. Since the chemical characteristics of the residues are directly related to those of the incoming refuse stream, it is important to identify the major sources of leachable metals in the waste. The handling and disposal of sewage sludge can make up about half the annual costs of municipal wastewater treatment. The disposal problem is expected to grow with expanding world population and a subsequent increase in sewered areas. The principal methods of sludge management now in use are: agricultural/land application, composting, landfill disposal, ocean disposal, and thermal treatment.
Contamination of sludge with heavy metals, organic compounds or pathogens, and odours are potential problems in all these methods (Stegemann, 1992). Thermal treatment is becoming increasingly popular due to technological advances and the increasing costs of other methods. Although an ash residue is generated, the volume to landfill is decreased by approximately 85% by weight, compared to dewatered sludge. If sludge ash can be utilized, e.
g., as a construction material, land filling is no longer needed. Treatment before incineration and incineration conditions can have a pronounced effect on ash quality.
Pretreatment may involve adding ferric chloride and lime or organic polymers as flocculants. The dewatered sludge cake is usually incinerated at operating temperatures ranging from 800 to 900°C. Temperatures higher than this sometime lead to partial fusion of the fine ash particles to form clinker.
For multiple-hearth incinerators, a granular bottom ash is collected from the grate, and a very fine fly ash may be removed from the stack gas by an air pollution control system. Fluidized bed facilities produce only a very fine fly ash. Sewage sludge ash may be brown or reddish in colour and has a sandy texture, varying from extremely fine to coarse. Its bulk density and specific gravity are lower than those of sand. Although it is generally obtained in finely divided form with a combined silica, alumina, and lime content exceeding 60%, the material lacks pozzolanic activity. In most regulatory jurisdictions, the ashes are classified as nonhazardous (Stegemann, 1992).
Particle size distribution is not consistent from one sludge ash to another. Some ashes fall within the grading for fine aggregates. Some “fines” content of sludge ash can increase the water-holding capacity of the material and pose problems. In cementitious binders, an aggregate with a high water adsorption capacity causes reduced strength, decreased workability, and difficulty in proportioning mix quantities.
Information on composition is useful in assessing the potential environmental impact and effect of sludge ash on different end uses, but these effects depend on the speciation of metal contaminants present in sludge ash, and are best examined through practical tests of leachability and behaviour in the field (Stegemann, 1992). Sludge ash can be used in a variety of building materials, ranging from straight backfill to low-strength, controlled-density fill materials, to relatively high-strength concretes, and fired bricks and tiles. It has also been utilized with some success for the production of lightweight aggregates for use in concrete.
For concretes and mortars, results have been reported for diverse tests including: absorption, time of set, shrinkage, slump, compaction, pozzolanic activity, soundness, fineness, permeability, and compressive strength at various ages. It was found that sludge ash may be substituted for aggregate in mixes of up to 10% by weight without significantly affecting concrete properties. Processing sludge ash into lightweight aggregates for the concrete industry appears to be the most promising utilization option. Lightweight aggregate may be produced by firing pelletized or slabbed sludge ash (often mixed with clay) at 1100 to 1200°C to produce a hard, ceramic-like material, which can later be crushed, sized, and graded to conform to required aggregate specifications. The low thermal conductivity and high fire resistance of the aggregates make them suitable for use in thermal insulation and fire protection applications such as fireproof wallboard (Stegemann, 1992). Other potential uses include decorative gravel, backfill material, and hydroponic media. Sludge ash may also be used as inert filler in bricks.
Additions of 10% sludge ash by weight do not appear to significantly affect the engineering or visual properties of the bricks. Currently, the major impediments to sludge ash utilization are the existence of low-cost landfill disposal and resistance on the part of a conservative construction industry. Soil-and-plant filters can be adapted for sewage treatment in semi arid climates. This is a promising approach in those cases where sewage is not treated, or cannot be treated for any reason, before being discharged into a lake or river. A variety of low-cost sewage treatment plants have been developed and are known as root-zone systems, solid and plant filters, or vegetable sewage treatment systems. In these, not only the operating costs are quite low but also the level of efficiency of reduction of phosphates and nitrates (tertiary treatment) is fairly high. These approaches also lead to significant removal of industrial toxins. The design of these systems is basically an optimization of the well known field irrigation systems that used to be in vogue a century ago.
The difference is that the direction of flow is changed to horizontal instead of vertical and also the type of plants used are those that are well-adapted to soils affected by sewage irrigation. Phragmites communis, Scirpus spp. and Typha spp. are typical examples of such plants.
The role of the soil substrate is al more important today. The rhizosphere effect due to excretion of organ acids by plant roots is better understood, and hydraulic conductivity has been improved by root development. On the basis of modern knowledge, a good fixed-bed reactor with high biological activity, capable of treating over 30-40 times more of sewage in the same area without contaminating the groundwater has been designed. This type of reactor has been found to be quite good in reducing coliforms to the level of that acceptable for bathing water, viz.
, about 107100 ml of treated water.