The previusly discussed the inherent abilities of certain kinds of soil microbes to remediate a wide range of contaminants, either in an unmodified form, or benefiting from some form of external intervention like optimisation, enhancement or bioaugmentation. Unsurprisingly, some approaches to sewage treatment over the years have sought to make use of this large intrinsic capacity as an unengineered, low-cost response to the management of domestic wastewaters. Thus, treatment by land spread may be defined as the controlled application of sewage to the ground to bring about the required level of processing through the physico-chemical and biological mechanisms within the soil matrix. In most applications of this kind, green plants also play a significant role in the overall treatment process and their contribution to the wider scope of pollutant removal is discussed more fully in the next.
Although it was originally simply intended as a disposal option, in a classic case of moving a problem from one place to another, the modern emphasis is firmly on environmental protection and, ideally, the recycling of the nutrient component. The viability of land treatment depends, however, on the prevention of groundwater quality degradation being afforded a high priority. In the early days of centralised sewage treatment, the effluent was discharged onto land and permitted to flow away, becoming treated over time by the natural microbial inhabitants of the soil. This gave rise to the term a ‘sewage farm’ which persists today, despite many changes in the intervening years. Clearly, these systems are far less energy intensive than the highly engineered facilities common in areas of greater developed urbanisation.
The most common forms of effluent to be treated by land spread, or the related soil injection approach, are agricultural slurries. According to the European Com-mission’s Directorate General for Environment, farm wastes account for more than 90% of the waste spread on land in Europe and this is predominantly animal manure, while wastes from the food and beverage production industry form the next most important category (European Union 2001a). Removal of the constituent nutrients by soil treatment can be very effective, with major reduc-tions being routinely achieved for suspended solids and BOD. Nitrogen removal generally averages around 50% under normal conditions, though this can be significantly increased if specific denitrifying procedures are employed, while a reduction in excess of 75% may be expected for phosphorus. Leaving aside the contribution of plants by nutrient assimilation, the primary mechanisms for pollution abatement are physical filtration, chemical precipitation and microbiological metabolism. The latter forms the focus of this discussion, though it should be clearly understood that the underlying principles discussed in the preceding remain relevant in this context also and will not therefore be lengthily reiterated here.
The activity is typically concentrated in the upper few centimetres of soil, where the individual numbers of indigenous bacteria and other micro-organisms are huge and the microbial biodiversity is also enormous. This natural species variety within the resident community is fundamental to the soil’s ability to biode-grade a wide range of the components in the wastewaters applied to it. However, it must be remembered that the addition of exogenous organic material is itself a potential selective pressure which shapes the subsequent microbial comple-ment, often bringing about significant alterations as a result. The introduction of biodegradable matter has an effect on the heterotrophic micro-organism popula-tion in both qualitative and quantitative terms, since initially there will tend to be a characteristic dying off of sensitive species. However, the additional nutri-ents made available, stimulate growth in those organisms competent to utilise them and, though between influxes, the numerical population will again reduce to a level which can be supported by the food sources naturally available in the environment, over time these microbes will come to dominate the community. In this way, the land spreading of wastewater represents a selective pressure, the ultimate effect of which can be to reduce local species diversity. Soil experiments have shown that, in extremis, this can produce a ten-fold drop in fungal species and that Pseudomonas species become predominant in the bacterial population (Hardman, McEldowney and Waite 1994).
With so high a resident microbial biomass, unsurprisingly the availability of oxygen within the soil is a critical factor in the efficiency of treatment, affecting both the rate of degradation and the nature of the end-products thus derived. Oxygen availability is a function of soil porosity and oxygen diffusion can con-sequently be a rate-limiting step under certain circumstances. In general, soils which permit the fast influx of wastewater are also ideal for oxygen transfer, leading to the establishment of highly aerobic conditions, which in turn allow rapid biodegradation to fully oxidised final products. In land that has vegeta-tion cover, even if its presence is incidental to the treatment process, most of the activity takes place within the root zone. Some plants have the ability to pass oxygen derived during photosynthesis directly into this region of the sub-strate. This capacity to behave as a biological aeration pump is most widely known in relation to certain aquatic macrophytes, notably Phragmites reeds, but a similar mechanism appears to function in terrestrial systems also. In this respect, the plants themselves are not directly bioremediating the input effluent, but acting to bioenhance conditions for the microbes which do bring about the desired treatment.