Potential Applications of Industrial Commodities Derived from Sludge Treatment

Treated sludges are used beneficially in land application for agriculture as fertilizers or soil conditioners. These practices have been going on for cen turies. Other unconventional uses of sludge products have also been vigorously explored around the world. This phenomenon is more prominent outside the United States of America where land available for sludge disposal is limited, and ocean disposal of sludges are banned in European Union nations. Many proposals of potential uses of sludge products have been put forth, and some of them have been tried in laboratory or in commercial-scale operations. For example, dewatered treated sludges have been used successfully for manufacturing building materials, such as concrete and bituminous mixes, and also as a road subsoil additive utilizing chemical fixation processes (Aziz and Koe, 1990). The chemical fixation process involves combining treated sludge with stabilizing agents, such as cement, sodium silicate, pozzolan (fine-grained silicate), or lime, to chemically react with or encapsulate sludge particles (Metcalf and Eddy, Inc., 1991). The process produces a product with high pH that inhibits viruses and bacteria, and for many chemical treatment products, the product has the consistency of natural clay. Final residuals of incineration or other thermal processes have also been used to generate road subbase material or concrete aggregate (Takeda et al., 1989). Since the main ingredients for Portland cement powder are limestone, clay, silica, iron, etc., and incinerated sludge ash usually contains the same ingredients as clay, it has been successfully used as a part of the cement materials. Pulverized sludge ash, limestone, and dewatered sludge/clay slurries have been used successfully in lightweight concrete applications without influencing the product's bulk properties (Tay and Show, 1991, 1993). Sludge-based concrete has been deemed suitable for load-bearing walls, pavements, and sewers (Lisk, 1989). The cement industry is highly energy-intensive; however, the large energy costs of creating clinker (powdered cement produced by heating a properly proportioned mixture of finely ground raw materials in a kiln) at 1500°C can be offset by utilizing sludge as a low-cost and readily available supplemental energy source, depending on the percentage of volatile solids in the sludge. Furthermore, sludge can be injected into the exhaust gas chamber to eliminate NOx emissions using heat of the hot exhaust gases reacted with ammonia contained in the sludge to convert NOx to nitrogen gas (Kahn and Hill 1998).

Solidification of hazardous materials and heavy metals has long been an effective method to prevent harmful materials from leaching into the environment when these materials are disposed. This process can also be used for production of sludge-based products. However, the sludge from a food processing plant is unlikely to have these metals or other inorgan ics that are necessary to produce these products. Because some streams of food and agricultural wastewater have been discharged into a municipal sewage system (maybe after some forms of treatment to reduce BOD5 and TSS), the description below has certain relevancy. It is possible to incorporate sludge enriched with heavy metal into the manufacture of biobricks. In this approach, incinerator sludge ash is used as a clay substitute during the manufacture of these bricks. The process is said to improve the ceramic properties and product strength of the resulting construction materials (Anderson et al., 1996). It is reassuring to know that these biobricks do not release heavy metals during firing in the production or weathering in use (Alleman et al., 1990). Additional benefits of the biobrick technologies include volume reduction and substantial savings on water and fuel consumption as well as treatment costs. In Tokyo, Japan, a product called Metro-Brick made of 100% incinerated ash through mechanical compression under high temperature (1,050°C) has been used as pavement materials for sidewalks, community roads, public open spaces, and parks. Attempts have also been made to use sludge as an "activated carbon" for odorous gas treatment via adsorption and for flue gas treatment via desulfurization (Krogmann et al., 1997). Palasantzas and Wise (1994) investigated the possibility of producing calcium magnesium acetate using residual biomass from sewage sludge. It is reported that this technique would generate an overall cost savings of 68% over conventional disposal costs.

A technology called sludge-to-fuel (STF) utilizes the volatile solids in many biosolids for producing combustible oil. STF involves a process that converts sludge organic matter into combustible oil using a solvent under atmospheric pressure with temperatures ranging from 200-300°C (Millot et al., 1989). Alternatively, STF using hydrous pyrolysis can produce combustible oil under high pressures in the range of 10 MPa and high temperatures (Itoh et al., 1994). One STF system employs a hydrothermal reactor to convert mechanically dewatered sludge to oil, char, CO2, and wastewater. The char, making up 10% of the product, is sent to a landfill, the wastewater returns to the wastewater treatment system, and the gaseous emissions are treated and released to the atmosphere. The produced oil has approximately 90% of the heating value of diesel fuel and can be sold to off-site users or refineries (Hun, 1998). This is an example of thermal depolymerization to transform reduced complex organics to oil.

Other STF processes produce oils from sludge by employing activated alumina pyrolysis of digested, dried sludges, or toluene-extracted sludge lipids (Abu-Orf and Jarnrah, 1995). In either case of the sludge sources, sludge-associated metals seem to bind to the residuals, with final product conversion efficiency being dependent on the sludge particulate size, temperature, and heating rate (Takeda et al., 1989). Metals in sludges tend to be trapped in the residual of STF process and, thankfully, organochlorine compounds that survive treatment within a typical sewage treatment plant are likely destroyed in the STF processes (Bridle et al., 1990). Oils produced with the STF technology have the potential to be used as heating oil and possible chemical feedstocks (Boocock et al., 1992). Much lipid-rich sludge from mechanical food wastewater treatment plants appear to be a good candidate for the STF technology.

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