Short Rotation Forestry Using Sewage Sludge and Biosolids Implications

Short-rotation forestry (SRF) is advocated as a means of phytoremediation and for disposing of sewage sludge [157]. Fast growing trees such as Salix (willow), Populus (poplar), Alnus (alder), and Eucalyptus (eucalypts) are ideal trees for SRF and have gained importance in mitigating CO2, industrial effluents, and other air pollutants. Large quantities of fertilizer and water are required to sustain the intensive management of SRF. This demand prompted the application of sewage sludge, with the additional benefit of providing a viable alternative for the waste disposal [157-160].

SRF has been successful in Sweden for the production of biofuel for the past 20 years. SRF takes its name from the short rotation time typical of poplar and willows (4 to 6 years) compared to conifers (60 to 120 years) or broadleaved trees (35 to 50 years). Under optimum conditions (i.e., intensive management strategies), SRF can yield between 12,000 and 20,000 kg dry matter (DM) ha-1 yr-1, although in general, lower yields of between 6000 and 9000 kg DM ha-1 yr-1 are more likely. If SRF is used for the generation of energy, approximately 4.5 MWh of energy can be produced from burning 1 ton of dry matter (50% moisture) [158].

A substantial body of data now indicates that biomass plantations are irrigated with sewage effluents; this helps in recycling effluents, biosolids, and other wastes. Current research on the use of sludge as a fertilizer for SRF indicates strongly that willow plantations may be able to perform the majority of the required cleanup steps well. SRF could be used as "vegetation filters" to utilize the nutrients in municipal sewage sludge, wastewater, leakage water, and bioash (wood ash), although dredged sediments containing organic chemicals and pathogenic bacteria have also been effectively "purified" following application onto biomass plantations [160, 161].

The tree species used in biomass forestry vary between areas in which they are grown. Many of the SRF species commonly used, especially Populus spp., have also shown considerable potential in the remediation of groundwater contaminated with inorganic chemicals — specifically, the P deltoides x nigra and P. trichocarpa x deltoides clones. SRF fulfills important environmental and ecological factors: there is no net CO2 contribution to the atmosphere compared to energy production using fossil fuel; in fact, there is a small net uptake. Pesticide use is lower than on conventional agricultural crops, and the growth of willows for SRF improves the condition of the soil as well as biodiversity

Primary requirements of SRF are a rich supply of nitrogen and water. The ability of fast growing trees to remove nutrients and water can also be applied to wastewater treatment; excessive concentrations of nutrient chemicals pose environmental problems when present in excess, such as eutroph-ication of streams and lakes, causing algal blooms, and a progressive reduction in potability. Standards usually monitor the biological oxygen demand (BOD), total N, ammonia N, and total P in water and often these strict regulatory limits cannot be met by conventional treatment methods alone.

The elimination of P from wastewater generally involves the use of large quantities of chemical agents, which are difficult to clean up once the process has been completed. Obarska-Pempkowiak [162] found that Salix viminalis and S. arenaria could significantly reduce the nitrogen concentration of municipal wastewater; in particular, S. viminalis reduced the total N concentration of wastewater from over 35 g m-2 to less than 5 g m-2 over a period of 270 days. Typical N and P contents of applied sludges are approximately 4.5 kg N t-1 and 2.8 kg P t-1 in the raw cake, of which 1.5 and 1.4 kg t-1 N and P respectively are available in the first cropping season. Willow and poplars irrigated with sewage sludge showed an increased yield and that the trees took up almost 95% of the N and P load of the effluent, simultaneously lowering the BOD of the effluent by an average of 96%.

Studies have emphasized that SRF can be used for "collecting" metals from the soil — in effect, using the trees as biological filters. Goransson and Philippot [163] found that almost all of the Cd applied during sludge application was taken up by Betula pendula and that, theoretically, the trees could remove 1.5 kg Cd ha-1 yr-1 when watered with sludge. Landberg and Greger [164] found that net transport of heavy metals to the shoots varied widely between 1 and 72%; therefore, certain clones were able to accumulate more metals than others, indicating the need for clone selection for resistant trees. The efficiency with which willows can remove metals from sewage sludge-fertilized soils has been demonstrated in several key studies.

Some evidence suggests that fast growing trees may be able to play a major role in reducing the concentration of organic chemicals in contaminated sludges, soils, and groundwater (see next section). Using land-farming techniques, Harmsen et al. used willows to remove PAHs from dredged harbor sediments and concluded that the high yield of biomass was instrumental to reducing the costs. Willows and poplar trees were also used in a multispecies wetland system to remove bacteria and other pathogens from municipal wastewater.

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