Environmental Biotechnology - Jordening and Winter

304 11 Bioreactors

Another example of composting contaminated soil (in a column) was presented by Gorostiza et al. (1998). In a soil column the degradation of pentachlorophenol with and without added compost was tested; compost addition significantly enhanced the degradation rate (residence time about 60 d).

On a larger scale (in Finland), composting systems have been implemented by mixing contaminated soil with spruce bark chips as a bulking agent, lime, and nutrients beforehand. The bed is constructed as a biopile and can be turned by a trac- tor-drawn screw mixer. Degradation temperature rose during composting to as high as 44 °C. During five months the mineral oil concentration dropped from 2400 to 700 mg kg–1 (Puustinen et al., 1995).

In another experiment, four biopiles of 10 m3 each were created to treat chloro- phenol-contaminated soil. Chalk, commercial fertilizer (NPK), and bark chips (as a bulk aeration agent) were added. After two months 80% of the contaminant was removed (Laine and Jorgensen, 1995).

11.4.3

(Semi)Continuous Operation: The Rotating-drum Bioreactor

In a rotating-drum bioreactor (or revolving tubular reactor), the reactor itself acts as a rotating conveyer in which the contaminated solids are mixed (Fig. 11.3). In a (semi)continuous operation the solids are transported (screwed) through the system. Loading at the head of the process is combined with unloading at the back. The solids residence time in the system is determined by the shape of the screw blades, the rotation speed, the scale, and the amount of material present. Experiments have been carried out in a bioreactor with a length of 25 m and a diameter of 3.5 m. A warm air blower provided an appropriate temperature and oxygen, a sprinkler system was installed and nutrients added; the residence time was 14 d. For starting concentrations of mineral oil contaminant ranging from 1000 to 6000 mg kg–1, at a temperature of 22 °C, the contaminant concentration decreased to 50–350 mg kg–1. Most breakdown activity took place in the first few days (Munckhof and Veul, 1990).

In a revolving tubular reactor developed by BioteCon in Germany, having a length of 18 m and a diameter of 2.5 m, between 0.5 and 1 t h–1 of soil can be treated (Kiehne et al., 1995). Model drums with diameters of 0.5 m and 1 m were tested. The typical moisture content was 15%, the revolution speed was 1 h–1, and the aeration rate 0.125 VVM. In the trials the breakdown of motor fuel, kerosene, and phenol was investigated. Under batch conditions kerosene was degraded from 8300 mg kg–1 to 500 mg kg–1 in 300 h. When the same soil was treated in a repeated batch (25% of the treated soil was returned and mixed with new soil), within 180 h end concentrations of 180 mg kg–1 were reached. The increase in microbial activity due to the reuse of biomass was considered responsible for the enhanced breakdown.

In an experiment a combination between heap leaching and reactor processing was tested. After treatment in the reactor for 120 h, kerosene-contaminated soil was deposited in a heap for four weeks. The treatment was finalized by a second reactor treatment for 100 h. With a total residence time in the reactor of 220 h, an overall conversion of 98.5% was achieved. It was concluded that for “contaminated soils in

11.5 Comparison of Bioreactors 305

the order of 500–3000 t the revolving tubular reactor especially demonstrates its advantage” (Kiehne et al., 1995). The technology has been developed to operate on-site.

Another solid-state process was developed by Umweltschutz–Nord (Bremen, Germany). Using a steel tube having a length of 45 m and an diameter of 3 m, the contaminated soil is screwed through the Terranox system. For mineral oil-contaminat- ed soils a treatment period of 6–8 weeks is used; the system can also be used for a combination of anaerobic and aerobic treatments.

On a smaller scale (3, 6, and 90 L) a blade mixing bioreactor was operated as a sol- id-state process (Hupe et al., 1995). The authors concluded that, in addition to the benefit of adding compost, moisture enhances the process; however, the water content should be <65% of the maximum moisture content to obtain solid-state processing conditions. The aeration rate is also a key parameter.

11.5

Comparison of Bioreactors

Table 11.1 shows a technological and economical characterization of the three bioreactor systems. It is clear that composting is the ‘low technology’ type of bioreactor, requiring only limited technological infrastructure and investment. Slurry process-

Table 11.1 Comparison of bioreactors.

306 11 Bioreactors

ing requires a technological infrastructure, and rotating-drum systems have an intermediate position.

Experiments show that during batch processing, the microbial breakdown slows down at low concentrations and full conversion is not reached. Continuous processing significantly improves the results.

The economic feasibility of bioprocessing depends largely on the nature of the waste streams to be treated. Basically, waste streams having a large water content and a considerable fine fraction (which is predominantly contaminated with organic components) are suitable for bioprocessing.

From the economic point of view, continuous slurry processing in an off-site installation can be beneficial only on a large scale. Capacities >40 000–60 000 t a–1 (depending on the local conditions) are needed to benefit from the economy of scale and thus to perform at acceptable cost levels. For batch-operated composting processes, smaller volumes, handled close to or on the site, may offer solutions if sufficient area is available and time is not limited. Rotating-drum systems in which sol- id-state processing is achieved are considered to perform well for smaller volumes (up to 3000 t) in a mobile plant used on-site.

11.6

Conclusions and Outlook

11.6.1

Conclusions

•In terms of product quality, energy consumption, and costs, bioprocessing has the capability to function as a useful waste recycling technology.

•Comparing the three bioreactor types considered, we can conclude that

–slurry processing offers the best option for rapid, controlled integral treatment

–composting can be used as a batch approach (biopiles) for smaller amounts

–rotating-drum bioreactors are a solid-state alternative to slurry processing

•In the decontamination market, bioreactor processing should focus on wet waste streams having a large content of contaminated fines; the contamination should be essentially organic in character.

•Unambiguous analytical techniques and procedures and clear (international) recycling standards are needed to properly evaluate the various technologies, the end products, and calibration standards.

•Research has to identify the reasons for the hampered breakdown as measured in various field experiments. Especially for large input concentrations, the bioavailability (or even toxicity) of the contaminants may seriously interfere with optimal microbial breakdown conditions. Ways have to be found to overcome these limitations.

•To fully explore the possibilities of bioprocessing, notice should be taken of the benefits of (semi)continuous processing.

314 12 In-situ Remediation

ter scale covers local aquifer systems. From scale to scale, different constraints limit the processes. In general, the constraints of the largest scale, i.e., hydraulic site conditions, might be the overall limiting processes. Hydraulically favorable conditions are offered by course sediments with a hydraulic conductivity of >10–4 m s–1.

Some of the main features of in situ technologies are to transport supplements (nutrient salts, electron acceptors or donors) to the contaminants and to remove reaction products, as well as to induce proper environmental conditions for contaminant degradation. This may, e.g., be implemented by pumping of reaction-pro- duct–enriched groundwater, water cleaning, and reinfiltration of water enriched with nutrients.

The efficiency of in situ biodegradation depends importantly on the distribution of the nutrients in suitable concentrations within the aquifer. Biodegradation processes that require aerobic conditions may be limited by the lack of oxygen. The maximum O2 concentration dissolved in water that can be infiltrated is given by its solubility of about 50 mg L–1. The oxygen supply can be increased by using hydrogen peroxide (H2O2), which is decomposed to O2 and water enzymatically or by geogenic components. However, the concentrations of H2O2 in the infiltration water are limited by its toxicity to bacteria at concentrations above 1000 mg L–1 and the possibility of O2 bubble formation, which would lead to a decrease in hydraulic conductivity and thus to decreased nutrient and oxygen supply.

The demand for oxygen or other electron acceptors decreases during the course of the remediation. In the beginning, electron acceptor consumption is usually limited by the supply rate. In a later phase, when all readily available compounds have been degraded, the electron acceptor consumption is limited by the rate of contaminant dissolution or diffusion out of micropores. Especially in this phase is it important, if O2 is added, to use proper O2 infiltration concentrations to avoid bubble formation.

The use of denitrifying conditions for contaminant degradation by the infiltration of nitrate may also lead to the formation of bubbles, because the end product of the denitrification is nitrogen (N2), which has a comparably low solubility. If nitrate is overdosed or the exfiltration rate is too slow, this may result in N2 degassing and bubble formation.

12.2 Investigations

Each site has to be investigated not only for its geological, hydrogeological, and contaminant situation, but also for the site-specific degradability. The following description refers to laboratory methods for investigation of the microbiological degradability of contaminants. These methods are divided into two stages [1] (Fig. 12.3). During the first stage the general degradability is determined, i.e., whether the contaminants at the given site are sufficiently degradable. Conclusive demonstration of the fate of the contaminants can be obtained only by using tracer (e.g., 14C-labeled substances). However, this is possible only in closed laboratory systems and not on a