Municipal solid waste incineration bottom ash as soil aggregate

      Nitrogen, phosphorous and potassium are the three main nutrients for plant growth. Both MSWI fly ash and bottom ash have been tested to provide nutrients to soil in agricultural applications. As MSWI bottom ash contains acceptable amounts of phosphorous and potassium, it can be used as a partial replacement of commercial fertilizers. In addition, the lime in fly ash can reduce the soil acidity, thus it can be used as a liming agent.

Source: http://plantnutrientsproject.weebly.com/nutrients-in-plants.html


     However, there are many restrictions for these applications. The heavy metals in bottom ash will be toxic to plants and animals; the high salts content will induce salt stress in plants; the pH value in soil will affect the mobility of elements; and the leaching of heavy metals into ground water will cause environmental concerns. As a result, more research studies have to be done in this field (Glordano et al., 1983).

Source: http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1864-564X/earlyview

     One of the studies showed that MSWI fly ash, bottom ash and combined ash can influence plant growth in a positive manner. Growth of alfalfa and Swiss chard in ash-amended soils was similar than that in soils amended with phosphorous and potassium fertilizer, indicating that MSW ash can supply essential nutrients for plant growth. However, the high Mo concentrations and uptake of Cd was significant enough to raise concern if the plant tissue was to be ingested by cattle, sheep, and swine. Moreover, the high soluble salt content can cause problems with sensitive plants under high amounts of ash amendments (Rosen et al., 1994).

Source: http://www.coal-ash.co.il/english/info_uses.html

     Waste ash also has a high pH and may be beneficial as a liming agent (Sander and Andren, 1997; Zhang et al., 2001a). It has been reported that in Japan the liming effect (its acid neutralization capacity) of incinerator bottom ash (IBA) was equivalent to 15–40% of calcium oxide, a very powerful liming agent (Zhang et al., 2001b). Therefore, it is practical to use municipal waste ash as a soil nutrient and as a liming amendment for acidic soils as a way to ease the burden of its disposal (Kawase et al., 1972). Waste ash can be applied by mixing it with other materials, such as animal excrement and urine, food scraps and sewage sludge (Zhang et al., 2001).

Source: http://www.gcsusa.com/technology.htm


    Caracteristics of waste ash are variable and depend on the materials burned; therefore, the application of waste ash to agricultural land requires evaluation on an individual basis (Japan Environment Agency, 1998). However, adverse effects can include phytotoxicity with excessive application. In the case of Taiwan, MSWI bottom ash may contain heavy metals and persistent organic pollutants, such as dioxins (TEPA, 2011). 

Source: http://rmrc.wisc.edu/wp-content/uploads/2012/10/asphalt_lab_samples08_22591-717x250.jpeg

     Therefore based on references, the present investigation attempted to identify the feasibility of using MSWI bottom ash and binary mixtures with green waste biochar as soil modifiers, since theoretically organic matter may adsorb heavy metals present in the MSWI bottom ash.

    In Taiwan, there are currently twenty-four municipal solid waste incinerators that generate approximately 940,000 tons of bottom ash per year (TEPA, 2011a). Moreover, the proportion of bottom ash generated is 15~20% of the total volume of municipal solid waste (MSW) incinerated.  The Environmental Protection Agency of Taiwan (TEPA) has reported that for bottom ash the rate of recycling stands at approximately 80% (TEPA, 2011b).

Bottom ash applications as a percentage of total reused

Source: http://rmrc.wisc.edu/ug-mat-coal-bottom-ashboiler-slag/

     This material is considered to have great potential for recycling (Van Gerven et al., 2007). In Taiwan the TEPA has reported that from a total of 2.15 million tons of bottom ash, 29% is recycled as coarse aggregate base, 5% as foundation and road fill, 28% as raw materials for the manufacture of controlled low strength materials, 27% as aggregate for non-reinforced concrete, 1% as asphalt concrete aggregate, 2% as brick aggregate and 8% as coarse aggregate for filling pipeline trenches (TEPA, 2011b).

Source: http://www.waste-management-world.com/articles/2011/10/recycling-waste-to-energy-bottom-ash-contributes-to-efficiency.html

     There is a crucial need to find an environmentally acceptable alternative to ash waste (fly ash and BA) disposal. Currently in Taiwan, incineration of municipal waste is one of the most used waste management options (Huang et al., 2006). In the same publication, the author noted that much effort has been undertaken in Taiwan in the past decade to reduce the quantity of MSW and improve its management.

Source: http://www.go.asia/case-study-taiwan/

     In 2008 a serious typhoon affected the rivers and mountainsides of Taiwan. Large amounts of fine sand were deposited along the banks of most rivers in the South of Taiwan (TEPA, 2009). This abundance of fine sand has depressed the market for MSWI bottom ash for four years, and it has been predicted that there is enough fine sand for the next 45 years (TEPA, 2011a). The use of BA on agricultural land offers an opportunity for the recapture of needed plant nutrients.

SOurce: http://www.jansengroup.com/en-us/products-services/incinerator-scrap

    Reuse of ashes could potentially reduce the need for application of commercial fertilizers, help to remediate acidification and help to solve the problem of ash disposal. A large body of previous studies has shown that ash wastes contain earth elements such as Si, Al, K, Ca and Mg, as well as micronutrients considered essential for plant growth (Theiss and Padgett, 1983; Lisk, 1988). Ash wastes typically have a high pH and may be beneficial as a liming agent (Sander and Andren, 1997; Zhang et al., 2001).  Therefore, the use of municipal waste ash as a soil nutrient and liming amendment for acidic soil may be practicable. However, there are limitations for these applications.  

     The heavy metals in bottom ash will be toxic to plants and animals; the high salts content will induce salt stress in plants; the pH value in soil will have an effect on the mobility of elements; and the leaching of heavy metal into ground water will cause environmental concerns. As a result, more research studies have to be done in this field (Lam et al., 2010). Recently, preparing biochar from biomass wastes has been considered as a possible alternative treatment method for agricultural residues, e.g., rice husks and bamboo residues (Major, 2009) and the potential to retain heavy metals could be an alternative for future treatments of bottom ashes.

Source: Source: http://www.sciencedirect.com/science/article/pii/S138917231300217X