Land Application of Sewage Sludge: Physicochemical and Microbial Response - Reviews of Environmental Contamination and Toxicology - page 43

world. Disposal alternatives frequently undertaken to remove such sludges have
included soil application, dumping at sea, land illing, or incineration (Sanchez
Monedero et al. 2004 ). In the United States, dumping of sewage sludge in the ocean
was banned as of 31 December 1991 (Hill et al. 1996 ; USEPA 1999a, b ) and in the
European Community in 1998 (Zhidong and Wenjing 2009 ). Land illing and land
application of sewage sludge are suggested to be the most economical sludge dis-
posal methods (Mc Grath et al. 1994 ; Metcalf and Eddy 2003 ). Land application of
sewage sludge is the most economical practice for reducing sewage sludge waste,
and this approach also offers the opportunity to recycle beneicial plant nutrients and
organic matter to soil for crop production (Laturnus et al. 2007 ; Singh and Agrawal
2008 ; Suhadolc et al. 2010 ; Silva et al. 2010 ). Assuming that sewage sludge has no
signiicant levels of toxic pollutants, its application on agricultural land has great
value, because of the potential it brings for fertilization and soil conditioning.
Our purpose in this review is to address the effect that land application of sewage
sludge has on soil physicochemical properties and on soil microbial response.

2 Characteristics of Sewage Sludge

The sewage sludges produced at different treatment plants and during different sea-
sons vary in physicochemical properties. As a result, knowledge of the chemical
composition of each kind of sewage sludge is necessary before it is used for land
application. The characteristic of sewage sludge depends not only on the nature of
the wastewater from which it comes but also on the processes by which the waste-
water is processed. Sewage sludge is generally composed of organic compounds,
macronutrients, a wide range of micronutrients, nonessential trace metals, organic
micropollutants, and microorganisms (Kulling et al. 2001 ; Singh and Agrawal 2008,
2009 ) (see Table 1 ). What humans use in their daily lives (e.g., insecticides, deter-
gents, pharmaceuticals, etc.) inds its way to either water or solid waste, and inally
reaches treatment plants. Waste from different household industries also contributes
heavy metals to waste water and, therefore, ultimately to sewage sludge (Singh and
Agrawal 2008 ). Some important sources of heavy metals that ind their way to the
wastewater stream and to sewage sludge are shown in Fig. 1 .
The macronutrients present in sewage sludge serve as a good source of plant
nutrients, and organic constituents impart beneicial soil conditioning properties
(Logan and Harrison 1995 ; Singh and Agrawal 2008 ). It is very rare that urban
sewerage systems transport only domestic sewage to treatment plants. Usually,
industrial efluents and storm-water runoff from roads and other paved areas are
also discharged into sewerage treatment systems (Singh and Agrawal 2008 ).
Therefore, sewage sludge may contain many different toxic materials (e.g., heavy
metals, pesticides, toxic organics, hormone disruptors, detergents, and various
salts), in addition to natural organic material (Mc Grath et al. 2000 ; Singh and
Agrawal 2008 ). Sewage sludge was collected and characterized from eight Indiana
cities in the USA over a 2-year period. Results showed that organic N and inorganic