Ex Situ Bioremediation Technologies

In many cases, it is necessary to move the contaminated soil or groundwater to a site where a suitable treatment system can be engineered. Contaminated soil may be excavated and moved to landfills; to thermal treatment systems, e.g., incinerators; or to a variety of bioremediation systems including biopiles, windrows for composting, landfarms, and soil slurry reactors. All have their merits. The choice of which technology to use often is driven by the required performance criteria, i.e., the nature of the contamination and the levels of cleanliness that must be achieved and the cost of remediation, including the cost of transporting the contaminated soil or water from the site of contamination to the site of treatment. Bioremediation technologies often can be performed at or very near the site of contamination, reducing the cost of transporting contaminated materials and thereby making the economics of bioremediation more favorable than the other physical disposal and treatment technologies. If the economic and technical analysis favors bioremediation and if a risk-based remedial design has concluded that ex situ treatment is the optimal approach, there are a variety of technologies available which can be considered.

Two technologies—biopiles and windrow composting—currently dominate the ex situ bioremediation market for treatment of contaminated soils. Both are aerobic processes in which the soil is excavated and heaped into a defined space for treatment. In composting, an organic material is added so that microorganisms generate heat through their metabolism, often causing the temperature to rise to at least 60°C (150). Except for the addition of organic material to support heat generation, biopiles and composting are essentially identical processes. Both technologies involve preparation of the contaminated soil to favor aerobic microbial metabolism of the contaminants; this may involve the addition of bulking agents, fertilizers, and water. Composting windrows for contaminated land bioremediation are aerated by periodic turning of the windrows with a modified windrow turner. In a biopile, aeration is accomplished through a network of slotted plastic pipes, either passively or by forced aeration. If space at a site is a constraint, then biopiles would be favored since a compost system requires sufficient space between windrows for access for the turning equipment. Biopiles can be formed with much larger volumes of soil than can be achieved for windrows, since the height of the windrow is limited by the size of the machinery used to turn it.

Biopiles and windrows have been used successfully at pilot scale and full scale for the bioremediation of a wide range of contaminants. While the majority of applications have been at petroleum hydrocarbon-contaminated sites (223, 339), they have also been used for manufactured gas plant sites (274), pharmaceutical wastes (118), chlorophenols (167), creosote (containing high concentrations of polynuclear aromatic hydrocarbons [PAHs]) (22, 65), pesticides (206), polychlorinated biphenyls (PCBs) (205), and nitroaromatics (46). Box 5.1 presents an example of a large-scale application in which biopiles were successfully employed as part of a large-scale bioremediation effort in Italy.

An emerging market for ex situ bioremediation by biopiles and windrows is cold-climate cleanup of fuel and oil spills. Crude oil is generally more persistent in Arctic tundra than in other regions, and a number of oil fields are situated in cold regions, e.g., in Alaska and Siberia. The feasibility of Arctic tundra ex situ bioremediation has been demonstrated (212), and the need for temperature, nutrient, and moisture control makes ex situ bioremediation the likely technology of choice in the Antarctic (7). These technologies appear to be applicable to arid desert contaminated sands also (31).

The third generic choice of ex situ technology is landfarming, which is more of a niche choice that has been used in the oil industry, from such diverse climates as Canada to Bolivia (371) to Saudi Arabia (126, 127), for decades for the treatment of refinery residues. It is a shallow treatment that is land-intensive, which makes it unpopular for most applications, particularly inner-city brownfield sites, typified by former gas stations and sites with leaking underground storage tanks (USTs).

The other generic technology is the use of slurry bioreactors, which offer quite different advantages over the other technologies, principally in the level of process control that is possible in a bioreactor and flexibility of operation. Combined slurry- and soil-phase approaches for bioremediation have also been investigated (224). In the following sections, we will discuss each of these approaches.

Biopiles

Biopiles range in size from sub-cubic meters for pilot-scale investigations to 10,000 m3 at full scale (163). Design is critical for achieving optimal performance, and remediation contractors have their own proprietary designs and engineering specifications for biopiles. In this discussion, design and engineering aspects will be considered in general terms with reference to real examples. The discussion will focus on the application of temporary biopiles, which are widely used around the world, as opposed to permanent installations, which have achieved success in the United States but are far less popular in Europe. The design and engineering considerations are similar but differ significantly with respect to the construction of the base upon which the contaminated soil is piled. Permanent installations usually have a

BOX 5.1 Biopile Treatment of Oil-Contaminated Soils in Italy

As a result of the Trecate 24 blowout that occurred in the course of drilling an oil exploration well in northern Italy, a very large area of highly productive agricultural land was contaminated. Biopiling and landfarming were successfully employed to provide an effective, integrated approach to petroleum hydrocarbon remediation. The unrecoverable crude oil impacted approximately 1,500 ha of superficial soils that were used primarily to cultivate rice, the vadose zone, and the underlying aquifer. To return the fields to production, the well's owner initiated a $45 million bioremediation project; bioremediation was used because of the need to remediate the valuable rice production soils without altering their pedelogic and rice production capacity (262).

Highly impacted soils (-5,000 to >100,000 mg of total petroleum hydrocarbon kg"1) (approximately 12.5 ha with a volume of 26,000 m3) were treated by using two biopiles, each approximately 135 m long and 50 m wide. The biopiles had an air injection system to provide oxygen to support aerobic hydrocarbon biodégradation, a moisture and nutrient delivery system to provide water and nutrients, a heat trace system to optimize temperature to ensure maximum biological activity, and a system of probes and sensors to monitor internal process control parameters. Biopile hydrocarbon biodégradation rates as high as 120 mg kg-1 day-1 were achieved.

concrete base, whereas temporary biopiles make use of different types of plastic liners.

Effective biopiles have been constructed in a large variety of shapes and sizes. There are no guidelines or limits for height or width of biopiles, but it is wise to build them such that the maximum reach of the front-end loader being used to form the piles is not exceeded. If this guidance is not heeded, then the frontend loader will inevitably ran over the previous lift when adding the subsequent lift, thus compacting the soil and undoing the careful work previously done in soil preparation. In the early stages of construction, this might also destroy the piping runs at the lower levels in the bio-pile.

It is important that the shape and size of the pile should be considered as a means to creating conditions of even aeration throughout the pile. Sides that do not have a high slope might lead to overaeration of peripheral soil relative to soil in the core. This can also lead to "wasted" aeration, in that air is lost from the sides, which might have the added consequence of removing volatile contaminants with it, creating unwanted volatile organic compound (VOC) emissions and aerosols, and cause preferential drying of the peripheral soil if the pile is not covered. In practical terms, shallow sloping sides also waste space and complicate maneuvering with a front-end loader.

Pile height is again governed by aeration. The very simplest biopiles have no forced aeration and rely largely on temperature gradient-driven convection to create airflow through the slotted pipes. This limits the size of the piles considerably. Forced aeration, either by blower or vacuum pump, relieves this restriction. Experience has shown that a single piping layer close to the base of the pile is sufficient for piles up to 3 m in height (349). Electing to build pipes higher than this would necessitate adding a second layer of pipes, which greatly complicates the construction.

The width and length of the biopile are determined by the total volume of soil to be treated (after amendments) and the amount of space available on-site. For large sites, it is com mon practice to build multiple biopiles. They may all be identical or may be treating soils that have been identified as more or less contaminated. The equation for the volume of a biopile based on the above geometry is

Figure 5.1 shows how to derive the volume of a biopile from some known dimensions. Alternatively, given a volume of contaminated soil, the equation can be used to calculate the possible dimensions of a biopile relating to the amount of space available at a particular site. By assuming a contaminated soil density of 1.5 tons m~3, sizing of a site for a known tonnage can be done, although greater accuracy would be achieved by measuring the density of the contaminated soil.

Biopiles have a variety of space requirements so that the site must be considerably larger than the biopile itself. A soil storage space is required to stockpile soil before processing. The size depends on the total volume of soil to be processed and the coordination with soil processing. It is quite feasible on a large site that a relatively small stockpiling area can be used since stockpiling, soil processing, and biopile construction can proceed at the same time. The majority of the soil processing space is required for stone removal, soil sieving, and shredding. A tank and manifold system may be required for leachate treatment. Another tank for water and/or nutrient supply may be needed. A secure container can be used to house blowers, with diesel generators kept close by outside. The container can also house spare parts, sampling equipment, and other sundry equipment. All of these have to be arranged logically on a site. At an early stage of site design, the turning circle of mobile plant, such as front-end loaders, tractors and trailers, and telescoping fork trucks, should be considered, especially in the critical areas of soil storage and processing. These should be located close to the biopile areas to maximize the efficiency of use of soil processing equipment. It is desirable to have the access road as short as possible and close to the soil processing area.

a w a

FIGURE 5.1 Calculation of volume of a biopile. After the work of von Fahnestock et al. (349). V = 1/6 h(Bl + 4 M + B2), where Vis the volume of the pile (in cubic meters), h is pile height (meters), B1 is the area of the lower base (square meters), B2 is the area of the upper base (square meters), and M is the area of the biopile midsection (square meters).

FIGURE 5.1 Calculation of volume of a biopile. After the work of von Fahnestock et al. (349). V = 1/6 h(Bl + 4 M + B2), where Vis the volume of the pile (in cubic meters), h is pile height (meters), B1 is the area of the lower base (square meters), B2 is the area of the upper base (square meters), and M is the area of the biopile midsection (square meters).

B1 = (/ + 2 a)(w + 2a) + hv + 2 aw + 2 al + 4a2. B2 = Iw.

M = [1 + 2(a/2)] X [w + 2(a/2)] = Iw + la + aw + a2.

V = l/6h(lw + 2 aw + 2 al + 4 a2 + 4 Iw + 4 aw + 4al + 4 a2 + Iw). = l/6h(6lw + 6 aw + 6 al + 8a2). = h(lw + aw + al + 1.33a2). tan 0 = h/a => a = h/tan 0.

An angle 0 of 50 to 60° gives an approximate slope (h/a) of 1.2 to 1.75.

Soil sieving and stone removal equipment can be brought in on hire for short periods; the awkward size and shape of this equipment necessitate that the access make it easy to maneuver.

All of the space at a biopile treatment facility needs to be secure from vandalism. Access to the site for workers should be through a single entry point, where clothes can be changed. The idea of a "clean" and "dirty" side at the entry point reinforces the idea that a biological facility is being entered. The housing for this can also be used to store other materials deemed necessary for health and safety and emergencies, e.g., respirators, disinfecting solutions, and trays. The site office should be located on the clean side, and its size and facilities depend on the size of the project and the number of stalf assigned to it on a full-time basis.

As a very rough estimate, excluding the site offices and access road, a site treating 1,000 m3 employing biopiles would occupy about 2,500 m2. On larger sites, economies of scale may be achieved since multiple biopiles should not require multiple stockpile and processing areas, although sufficient thought has to be given to the spacing between the biopiles for access for mobile plant. It should also be borne in mind that each contaminated site is unique, and site design is often constrained by the overall site size and shape. For example, a complex 4-ha site in the United Kingdom treating 34,000 m3 of soil contaminated from coal coking operations by biopiles requires two full-time staff members, the site engineer and site scientist. The site engineer is responsible for contractual work, liaison with the regulator and clients, and dealing with subcontractors, and the site scientist takes all samples, performs field tests, and does liaison with analytical laboratories. The biopile formation and site closure stages require more staff, but once a project is running it is not labor-intensive.

BIOPILE COMPONENTS The essential components and features of a biopile are shown in Fig. 5.2 (163, 349). The stylized geometry of a biopile is a trapezoidal encapsulated soil pile, with appropriate amendments, sitting on a base liner system (96). The biopile design includes piping for aeration and optional other design components, which may include an irrigation system and cover.

Biopile FigureBioremediation Pile
FIGURE 5.2 Elements of a biopile. After the work of Kodres (163). (A) Schematic. (B) Detailed diagram.

stalled on top of the liner. Slotted plastic pipes of various thicknesses and materials are available as the main element of the aeration system within the pile. Polyvinyl chloride (PVC) slotted pipe (2- to 4-in. diameter) is a common choice (Fig. 5.4). The pipes are embedded within a highly permeable matrix, such as new wood chips or gravel, to act as an aeration manifold, whether operated with a blower or a vacuum pump. The depth of this layer is variable but obviously the pipes must be sufficiently covered to minimize short-circuiting from them.

The length of the biopile determines the length of each aeration pipe run. At the manifold header side, each aeration pipe run starts with a solid, not slotted, length of pipe of the same diameter as the slotted pipe. This is typically of the order of 3 m long, the actual length dictated by the distance to the aeration manifold. It should proceed about 3 m into the pile before it is joined to slotted pipe to ensure that short-circuiting does not occur right at the start

Biopile Base. The biopile base should be built on a relatively solid surface. At its most engineered, the base consists of a soil or clay foundation (up to 25 cm), an impermeable liner, and a bund to contain leachate (Fig. 5.3). The biopile base should have a slight slope of 1 or 2° to allow drainage of leachate to an appropriately sited leachate collection sump located at a corner of the biopile. The impermeable liner, usually clay or a synthetic material, is then placed over the base. Clay liners are not recommended for highly soluble contaminants such as phenol. Synthetic liners of a high-density polymer are recommended, with thicknesses from 40 to 80 mil (1 to 2 mm). High-density polyethylene (HDPE) with heat-welded seams is ideal for this purpose. Thinner liners should be capable of taking the weight of heavy, even-tracked plant without tearing. Clean soil can be compacted on top of the liner to further protect it.

Aeration System. Once a biopile base has been constructed, the aeration system is in

Situ Bioremediation
FIGURE 5.3 Photograph of a biopile base showing its preparation. Courtesy ofWSP Remediation Ltd., Cardiff, United Kingdom.

of the pile. Next, the solid pipe is connected to the slotted pipe with a rubber or plastic connector. The length of the slotted pipe is dictated by the length of the biopile and, as at the start of the pipe run, should terminate some 3 m short of the far end of the biopile to prevent short-circuiting through the sloping edge. The slotted pipe is terminated with an end cap.

The start of the pipe run is connected to the header manifold by a gate valve of appropriate diameter. When several pipe runs are used to aerate a wide biopile, the gate valves are used to equalize the flow of air through each pipe ran. The zone of influence of each pipe ran is influenced by rate of airflow and also by soil porosity. Too-rapid airflow causes drying of the pile and may drive off VOCs creating an environmental concern in the vicinity of the pile. It also uses excessive electricity. By using relatively low-power blowers, this problem can be circumvented. To ensure even aeration within the pile, then, it is necessary to add several pipe runs parallel to each other. Once the biopile is built, it is very difficult to influence the porosity of the soil. As a general rule, parallel pipe runs are spaced about 2.5 to 3 m apart. Air velocity can be easily measured in each pipe run.

Connection of the header manifold to the blower then completes the system. A low-power centrifugal blower (Fig. 5.5) is sufficient for aerating large volumes of soil, and it is wise to install several small blowers of various powers (1 to 5 hp). Varying the rate of aeration over the duration of a project makes sense, as the need is greater at the start when there is a high level of contamination and microbial activity must be stimulated. Variable-speed blowers are therefore advantageous. It is not advisable to control aeration by on-off cycling, especially in the early stages, as even short periods with the blower off can lead to anaerobio-

Slotted Pvc Pipe
FIGURE 5.4 Photograph of a biopile slotted pipe (plastic pipe, 4-in. diameter).

sis in regions of the pile, and this is very difficult to monitor. As an average figure, the blower should be capable of delivering about 0.14 m3 of airflow per pipe run per min (130).

Depending upon the location of the project, operating in blower mode might have a high potential for stripping VOCs and creating an odor and health risk on and even off the site. Local legislation may then require containment measures that are rather expensive. In such circumstances, it is better to operate aeration by vacuum, as the captured air can be passed through a VOC removal system, typically granular activated carbon (GAC), and thus ameliorate the odor and risk. There has to be good justification for this, however, as operating in the extractive mode complicates the aeration setup. Pulling air through the pile will entrain condensate, and even leachate may be pulled through, so the vacuum pump must be preceded by water knockout and collection tanks. Granular activated carbon treatment of off-gases considerably increases the cost. It has been shown under laboratory conditions that VOC volatilization can be suppressed by the addition of activated carbon as a soil amendment (242). On a full-scale biopile, however, this would represent a large cost. If the extractive mode is deemed necessary, the pump should be capable of removing at least 15 pore volumes per day (131).

Passively aerated biopiles have also been used, in which a similar engineering system is used but without mechanical aeration. Airflow is created by differences in temperature between the soil and the outside air driving con-vective currents, facilitated by the slotted pipes, and by wind-induced pressure gradients. Naturally, this can achieve more limited aeration and might lead to uneven aeration as a result of a limited radius of influence as the air leaves the pipes and enters the soil mass. The central region of a pile would be particularly prone to local oxygen deficit (163). Wind-driven tur

FIGURE 5.5 Photograph of a typical blower for full-scale biopile operations. Courtesy of WSP Remediation Ltd.

FIGURE 5.5 Photograph of a typical blower for full-scale biopile operations. Courtesy of WSP Remediation Ltd.

bines have been investigated to see whether improved aeration can be achieved without electrical energy (176). Results were inconclusive, in that improved airflow was demonstrated but without evidence of enhanced bio-remediation.

Covers. Many bioremediation projects have been done without covering the biopiles, but covers offer some advantages. A primary advantage is that a cover prevents leachate formation and simplifies the biopile design accordingly. Another is that covered biopiles lose very little water. A typical ex situ bioremediation contract for petroleum hydrocarbon cleanup might last 3 or 4 months, during which time a covered biopile might lose only 1 to 2% of its initial water content. Thus, it would be sufficient to amend the water content during the construction phase only. If the alternative is to install an automated sprinkler or drip-type system to maintain water content within defined tolerances, then a cover is a much simpler engineering option. The moisture content, nevertheless, must be monitored since forced aeration, by either blower or suction, tends to remove moisture as the air entering the pile usually does so at less than 100% humidity.

The cover is often waterproof plastic sheeting, Visqueen, or a thin grade of HDPE liner, sufficiently thick to prevent tearing while being manually removed and refitted. If extractive aeration is being performed, completely impermeable liners may not allow sufficient air circulation. Framing systems have been tried but complicate the installation and are prone to water ponding and collapse. Alternatively, the pile can be covered with wood chips, which retain heat but allow air to flow.

Recently, fleece liners have been adopted from the composting of green wastes. Fleece liners are made from blown polypropylene.

They have properties similar to fleece garments. They resist moderate rainfall but are not completely impermeable to water and are also gas permeable. They allow some water to pass through to a biopile during operation, which will replace water lost through aeration. Gas permeability overcomes the problem with completely impermeable plastic covers mentioned above. Equipment is also available to mechanically remove and replace fleece liners (Fig. 5.6) from biopiles to cut down on labor requirements.

Irrigation Systems. If water has to be added, it is preferable to do it at the biopile formation stage if the batching procedures described above are being used. If an alternative water addition system needs to be employed, the preference is for a dripline irrigation system. Water flow from such a system can easily be monitored, the low rate of application prevents runoff, it can be set up to evenly irrigate a bio pile, and it can be operated without supervision. However, it is inevitably a further engineering complication that is best done without if possible.

BIOPILE FORMATION The standard way to form a biopile is to add all necessary amendments to the graded, sieved soil and then start lifting the soil onto the base and aeration system with a front-end loader. An alternative is to form the biopile with graded soil and any bulking agents and then add liquids at a later stage. The latter strategy carries risks in that it is difficult to achieve uniform distribution of materials. Usually, nitrogen and phosphorus are applied together. If a liquid source is sprayed onto the top of a biopile, the mobility of the nitrogen source will allow it to travel through the pile. However, phosphorus interacts with metals and other soil components and will be immobilized within a

Bioremediation Background
FIGURE 5.6 Photograph of a fleece roller. A self-propelled windrow turner can be seen in the background. Courtesy of Shanks Waste Management Ltd., United Kingdom.

meter of the top of the biopile. Also, if the occasion dictates the use of inocula, then very quickly the microorganisms used will be immobilized and achieving uniform mixing by application after biopile formation is all but impossible. Many laboratory and field trials have shown that bacteria do not move appreciably through soil (10) as a result of physical filtration by small soil pores and adsorption to soil particles.

Therefore, the safest way to guarantee the maximum level of homogeneity of materials in a biopile is to mix everything just prior to, or even during, biopile formation. The standard practice is to hire in soil grading (screening) equipment to remove stones and grade the soil down to an average particle size, usually 30 to 50 mm (Fig. 5.7). Parallel bar screens remove large stones, and then trommel or vibrating screens are used to grade the soil down to a smaller size. Large amounts of clay will require soil shredding as the next step in the process.

Knowing the capacity of the loader being used to feed the screening equipment and the speed of the conveyor belt, it should be possible to spray liquid amendments at the screening stage with some accuracy. It should also be possible to add bulking agents to a known percentage (usually by volume). If the soil requires shredding, then the effort will have been wasted. After screening and shredding, various equipments are available for soil mixing, and this is the optimum stage for adding amendments. The Kuhn Knight Reel Auggie (Fig. 5.8) is typical of U.S. equipment for this purpose, although it is designed for compost mixing. Contaminated soil may be batched with the other liquid and solid amendments, and the mixing action allows efficient aeration. A large, slowly rotating reel drives materials forward to

Bioaugmentation
FIGURE 5.7 Photograph of soil screening and addition of wood chips (the lighter material in the soil heap) and bioaugmentation culture (contained in the 1-m3 Intermediate Bulk Container), which is being applied by spray at the top of the soil grader.

FIGURE 5.8 Photograph of a Knight Reel Auggie for soil mixing and distribution. Courtesy of Kuhn Knight.

Kuhn Spray

FIGURE 5.8 Photograph of a Knight Reel Auggie for soil mixing and distribution. Courtesy of Kuhn Knight.

two blending augers. The combined action of the augers mixes, aerates, and discharges the materials, and the discharge can be done directly to a forming biopile or compost windrow. This equipment can be truck mounted or tractor mounted and can be driven electrically or by self-contained diesel engine. As soil is batched, it is easier to control the volumes and thus the final concentrations of any amendments.

In the United Kingdom, hiring a soil scree-ner is expensive, and its operation can force a reduction of activities on a small site. A relatively recent development has been the use of the ALLU bucket for the purpose of soil grading (Fig. 5.9). This equipment is able to crush stone, brick, and lightweight concrete while processing and aerating soil, and as the bucket size is known, amendments can be added accurately. It can be mounted on a variety of equipment, including front- and rear-end loaders and tracked and tire excavators. The high level of mobility means that mixing can be done directly onto a biopile or windrow, or it can be done direcdy onto a trailer. It has another convenient advantage in that the output tends to form soil piles in the shape required for windrows, so that subsequent manipulation to finish a biopile is easy.

Conventionally, a biopile is formed from back to front, the back being the end at which the aeration header is located. Common sense precautions to be taken are to make sure that previous lifts are not driven over, since this compacts the soil; driving over the aeration system will destroy it.

Windrow Composting

As described above, the main difference in materials used in windrow bioremediation and bi-opiles is that some organic, heat-generating material is added to the windrows. The objective is to increase the metabolic rate of indigenous hydrocarbon oxidizers. Most bacteria

FIGURE 5.9 Photograph of an ALLU bucket. It is fitted with rotating drums with blades, and also crushing bars, so that it can be used to grade soil containing materials such as brick. Courtesy of WSP Remediation Ltd.

FIGURE 5.9 Photograph of an ALLU bucket. It is fitted with rotating drums with blades, and also crushing bars, so that it can be used to grade soil containing materials such as brick. Courtesy of WSP Remediation Ltd.

grow over a range of approximately 40°C, whatever their optimum temperature for growth. Within the normal temperature range for growth of a bacterium, growth rate obeys the Arrhenius relationship between reaction rate and temperature. Increasing the temperature of a contaminated soil windrow from 20 to 30°C would be expected to more or less double the growth rate of hydrocarbon-oxidizing bacteria and therefore speed up the bio-remediation process. Another effect of increasing temperature might be to increase the bioavailability of poorly water-soluble contaminants since increasing the temperature should increase solubility, as well as decreasing viscosity of oily contaminants. During composting of contaminated soil, the thermophilic stage is not reached, and the temperature does not exceed 45°C (282).

In this regard, bioremediation of contaminated land by windrow treatment is a very dif ferent operation from composting of organic wastes or sewage sludge. The main difference between contaminated soil composting and traditional composting is that the former lacks the concentration of organic materials of the latter. A direct consequence is that heat generation is normally lower in contaminated land composting. For traditional composting of waste materials, the high temperature generated in the procedure is essential: several ordinances stipulate that every part of the compost must reach a temperature of 65°C at a water content of at least 40% for 3 consecutive days (143), for the purpose of pathogen kill. For contaminated soils, the likelihood of the material containing large pathogen loads is much less, and also high temperatures are not suited to the metabolism of most hydrocarbon-oxidizing bacteria. In addition, such high temperatures would involve water loss at a level that would complicate windrowing operations.

One of the purposes of turning windrows is to dissipate heat, and the increased porosity achieved by adding bulking agents aids heat dissipation.

WINDROW COMPONENTS A large variety of organic amendments has been used in composting bioremediation. Many are based on the application of manure, from either cows (22), pigs (368), or chickens (274). Sewage sludge is abundantly available globally, and it has been successfully used as an amendment in composting bioremediation (141). Virtually any putrescible material available in bulk can be used, such as vegetable wastes (22), spent mushroom compost (SMC) (95, 169), and even garden waste (118, 119, 205). The use of composting approaches to bioremediation of organic pollutants generally (282) and specifically the use of composting to treat PAHs (21) have been reviewed.

The use of SMC is an interesting case. SMC is the residual compost waste generated by the mushroom production industry. It is readily available, as mushroom production is the largest solid-state fermentation industry in the world; in the United Kingdom alone, there are some 400,000 to 500,000 tons of waste mushroom compost produced per annum (281). It consists of a combination of wheat straw, dried blood, horse or chicken manure, and ground chalk composted together. It is a good source of general nutrients (0.7% N, 0.3% P, 0.3% K plus a full range of trace elements), as well as a useful soil conditioner. A fascinating feature of SMC is that it may contain a relative abundance of extracellular ligninolytic fungal enzymes (169), which are relatively nonspecific in their substrate preference. Hence, they may assist in the biodégradation of aromatic molecules such as PAHs, giving SMC an additional role in composting bioremediation.

The construction of more-robust turning equipment, based on compost windrow turners but strong enough to turn large soil windrows, has allowed full-scale windrow bioremediation of contaminated land. Much of the discussion is similar to that for biopiling. For example, soil preparation is identical in the need for water, nutrients, and bulking agents, although the initial care in mixing need not be so rigorous since the whole point is that aeration is brought about by pile turning. The turning naturally mixes the contents of the windrow: the more often it is turned, the greater the homogeneity of materials, including moisture. Windrows are inherently simpler in design; therefore, this section will focus more on the differences between windrow composting and biopiling.

The natural shape of a windrow is more like a triangular prism than the characteristic trapezoidal, or incomplete pyramid, shape of the bi-opile. Windrows are suited to long, narrow sites, as they can be constructed to any length required (Fig. 5.10). Typically, windrows for bioremediation of contaminated soil would be of the order of 1.5 to 2 m high and perhaps 3 to 4 m wide. With the arrival of large, self-propelled windrow turners, windrows can now be constructed to greater heights and widths than before. Doing so necessitates more regular turning to prevent anaerobiosis, though.

Given that compost windrows can be much

FIGURE 5.10 Calculating volume of a windrow. Volume of windrow = 1/2 w X h X I. If the mass of contaminated soil and its density are known, the total volume of material, including the bulk materials such as organic compost and bulking agents, can be calculated. Knowing the width of the site, and the space between windrows, the number of windrows of set width can be calculated.

longer than they are wide, the small volume of material at each end can be ignored in sizing. That is why the volume of a triangular prism is a realistic choice.

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  • maria
    How often do you need to turn a biopile remediation?
    1 year ago

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