BIOREMEDIATION AND PHYTOREMEDIATION

BTY 884

Process analysis

Site characterization and feasibility studies must determine key chemical, physical, and microbiological properties of the site.

Site Characterization

Main objective of site characterization is to identify the contaminants, theirconcentration, and the extent of contamination.

The distribution of contaminants between soil and groundwater will largely determine whether soil or groundwater treatment is applicable, while the extent of contamination will largely determine the applicability of soil excavation and treatment

Physical properties of organic contaminants

Octanol/water partition coefficient is defined as the ratio of a compound's concentration in the octanol phase to its concentration in the aqueous phase of a two-phase system.

Measured values for organic compounds range from 10-3 to 107. Compounds with low values (<10) are hydrophilic, with high water solubilities, while compounds with high values ( > 104) are very hydrophobic.

Compounds with low water solubilities and high Octanol/water coefficients will be adsorbed more strongly to solids and are generally less biodegradable.

Highly soluble compounds tend to have low adsorption coefficients for soils and tend to be more readily biodegradable

Vapor pressures and Henry's Law constant, HA, measure liquid—air partitioning.

Henry's Law states that the equilibrium partial pressure of a compound in the airabove the air/water interface, PA, is proportional to the concentration of thatcompound in the water, usually expressed as the mole fraction, XA.

PA = HAXA

Aeration is often employed to strip volatile organic compounds from water andis favored by large HA values.

Conversely, volatilization must be controlled or contained in many bioremediation processes.

Henry's Law constant is highly temperature sensitive, and temperaturechanges of 10 °C can give threefold increases in HA.

In soil bioremediation the rate-limiting step is often the desorption of contaminants, since sorption to soil particles and organic matter in soils can determine the bioavailability of organic pollutants.

Bioavailability is also an important toxicity characteristic

Rates of in situ soil bioremediation are governed by mass transfer of contaminants(desorption and diffusion), the convective—dispersive flux of oxygen and nutrients,and the microbiological content of the soil.

On-site testing to determine the rate and extent of biodegradation can be done immediately in consideration of ex situ process feasibility

Desorption tests measure the site-specific soil/water partition coefficients for the contaminants of interest.

Measurements of aqueous phase and soil phase concentrations in equilibrium may be sufficient to indicate potential problems with soil sorption

Many organic contaminants are hydrophobic, have a low water solubility, and are tightly held to the soil phase.

Desorption of such contaminants is likely to be rate-limiting, especially (on ex situ bioremediation where addition of nutrients and microorganisms and careful control of environmental parameters can minimize these potentialrate-limiting effects.

Failure to bioremediate polyaromatic hydrocarbon (PAH) compounds has been attributed to strong sorption to soil

Surfactants and cosolvents has been investigated in attempts to increase bioavailability of contaminants that are strongly bound to soil.

High surfactant concentrations can be required to achieve small increases insolubility

2% surfactant solutions are needed to remove a high percentage of compounds such as higher-ringed PAHs, polychlorinated biphenyls (PCBs), and higher molecular weight hydrocarbons from soils.

High surfactant concentrations can be required to achieve small increases insolubility.

Microbiological characterization

Measurement of biodegradation rates by indigenous microorganisms is the firststep in microbiological characterization

These measurements can be complicated by low microbial populations or by the absence of species capable of degrading contaminants.

Also, optimum conditions of temperature, oxygen nutrient supply, and contaminantavailability due to low solubility and sorption can limit degradation rates, especially in early tests where these limiting factors are not well defined.

Main objective of microbial degradation tests is to determine whether theindigenous microorganisms are capable of bioremediation when conditions areoptimized, or if inoculation by nonindigenous microorganisms will be required.

Bioremediation sites contaminated by 2-sec-buty\-4,6- dinitrophenol (dinoseb), the first site was remediated with indigenous microorganisms, second site requiredinoculation with microorganisms from the first site

Environmental factors

Chemical analyses that support process design include measurements of pH, COD, TOC, nitrogen, phosphorus, and iron; and inhibitory, toxic, or essential metals

Soil type, clay and organic matter content, and particle size distribution analyses are used, for water, total suspended solids

Microbiological analyses supporting process design include BOD, plate counts, and shake flask and/or column degradation studies with indigenous microbes or introduced cultures

Bioremediation is usually carried out near neutral pH, although fungi often require an acidic environment

Most microorganisms are mesophilic, requiring temperatures in the25 to 37 °C range

More difficult to optimize are the oxygen and nutrient supply.

Most bacteria capable of degrading organic compounds are heterotrophic andrequire an organic compound as a source of carbon and energy

It may be necessary to add a readily metabolizable carbon source such as glucose to maintain cell viability or to increase cell growth and degradation rates.

Many xenobiotic compounds can be transformed by cometabolism, in which the transformation does not serve as an energy source for synthesis of cell biomass, which therefore requires a separate carbon source.

Sometimes concept of cometabolism is of limited use inbioremediation process analysis and design.

A chemical that is cometabolized at one concentration or in one environment may be mineralized under other conditions

Cometabolism can often be accelerated by the addition of a mineralizable compound with a structure analogous to the target compound.

This method of analog enrichment has been used to enhance the cometabolism of PCBs by addition of biphenyl, in which the unchlorinated biphenyl serves as a carbon source for microorganisms that cometabolize PCBs with the enzymes induced by biphenyl

Degradation is often coupled to growth and microbial mass, so the carbon source that best supports growth also gives the highest rate of degradationSince nitrogen and phosphorus supplies in soil systems are inadequate to supportmicrobial growth and degradation of organic compounds, most bioremediationprocesses supply these two compounds

Biodegradation can occur aerobically or anaerobically.

The most effective biodegradation of hydrocarbon compounds is mediated by aerobic bacteria.

Oxygen supply can be a rate-limiting factor, and large-scale aerobic bioremediationof hydrocarbons must include an aeration system as a critical design component.

Early bioremediation work focused almost exclusively on aerobic systems, butrecent improvements in anaerobic technologies have shown that a wide range ofnitroaromatic compounds, chlorinated phenols, PAHs, and PCBs can be degradedas well or better by anaerobic processes

In the selection of a microbial system and bioremediation method, someexamination of the degradation pathway is necessary.

At a minimum, the final degradation products must be tested for toxicity and other regulatory demands for closure.

Predicting degradation rate

Degradation rates and equilibrium properties are useful for processdesign.

During wastewater or soil treatment, target pollutants can be degraded ormineralized, volatilized, adsorbed onto effluent solids, or discharged in the liquideffluent.

Volatilization and sorption must be minimized so that the principal fate of contaminants in a bioremediation process is biodegradation

Table is a portion of the 196-compound data base used in a fate model toestimate volatile emissions, concentrations of toxic compounds in sludges, andremoval of toxic compounds during activated sludge wastewater treatment

The major error in predicting the fate of toxic compounds in wastewater treatmentplants was in estimating the value of the biodegradation rate constant.

For a compound such as tetrachloroethylene, with a low biodegradation rate constant and a high Henry's Law constant, the principal fate will be volatilization.

Published biodegradation rate constants do not provide a good comparative basisfor process design since experimental conditions vary greatly.

Many unreported factors can influence biodegradation rates, including transport effects, Acclimation of microorganisms to toxic chemicals, inhibition, and cometabolism.

Degradation rate constants are used mainly for order-of-magnitude estimates, and other approaches to predicting biodegradability have been developed that are based on physical or chemical properties of compounds.

A measure of hydrophobicity can be used to estimate relative biodegradationrates among homologous compounds.

Rates of anaerobic degradation of halogenated aromatic compounds arecorrelated with the strength of the carbon halide bond that is cleaved in therate-determining step

A promising method for predicting biodegradation rates is the group contributionapproach, which estimates biodegradability from the type, location, and interactionsof the substituent groups that make up a compound.

An experimental data base that assesses mixed culture biodegradation data for 800 organic compounds is available

Good predictions of degradation rates can be obtained for mixed-culture aerobic degradation in processes such as activated sludge treatment.

The activated sludge reactor can be optimized with respect to aeration using a model to calculate biodegradation and volatilization rates of organics as functions of aeration rate.

REACTOR OPTIONS

Reactor options are determined primarily by the physical properties of the wasteand the chemical and biochemical properties of the contaminants.

If the waste is found in groundwater, then a continuous supported reactoris desirable

A suspended batch reactor is preferable with contaminated soil

A polar target compound favors an aerobic reactor, while nonpolar compoundsfavor anaerobic reactors

Groundwater is easily treated in a continuous process in which the microbial biomass must be retained in the reactor by adhering to a support.

Soils are difficult to transport, so batch reactors are favored.

Agitation is a critical design parameter required for aeration, cell suspension, and transport of contaminants and nutrients during soil remediation.

Groundwater remediation

The technology for the removal of suspended and dissolved contaminants frommunicipal and industrial wastewater has been well established for wastes that arereadily biodegradable under aerobic conditions.

After preliminary treatment such as screening, grit removal, and sedimentation, a biological reactor degrades the organic matter in the wastewater during secondary treatment.

The reactor is an aerobic fixed film process or a suspended-growth, activated sludge process.

Industry relies heavily on treatment of their chemical wastes by publicly owned wastewater treatment facilities.

Removal rates of xenobiotic compounds are not well described

Activated sludge reactors

Polychlorinated phenols and biphenyls, phenols, phthalates, and PAHs weretested, with average removal rates of 97%. However, higher concentrations candestabilize an activated sludge system.

The presence of cyanide, pentachlorophenol, 1,2-dichloropropane, acrylonitrile, phenolics, and ammonia can cause instability in the operation of activated sludge plants

Mobile units are often required for the treatment of groundwater pumped out ofa contaminated aquifer, or wash water used to remove chemicals from acontaminated soil.

The advantages of an activated sludge design for treating high-organic-content wastewaters that produce a flocculating sludge are lost in these cases.

Low microbial growth, poor flocculation, and instability problems can make an activated sludge process very difficult to apply to groundwater treatment

Fixed film processes, similar to the trickling filter designs for wastewater treatment, can generally be operated at lower oxygen supply costs, with retention of bacteria in the reactor and a more stable operation.

Fixed film reactors

The BioTrol aqueous treatment system is a good example of a fixed film, continuousreactor used successfully for the treatment of groundwater.

This reactor is packed with corrugated polyvinyl chloride media on which abacterial biofilm is grown

The combination of groundwater flow, air sparging, and design of the support media facilitates the upward and lateral distribution of water and air in the reactor. Removal of more than 95% pentachlorophenol (PCP) is achieved at a 1.8 h residence time.

Fixed film or attached growth systems require degradative microorganisms with theability to attach to the surfaces of an inert packing.

Fixed film reactors can treat low concentrations of organics in wastewater because of biomass retention, and they can also treat concentrations as high as 1000 ppm.

The high biomass loadings of fixed film reactors render them insensitive to shock loadings, that is, to high fluctuations in organic loadings.

The outer layers of the biofilm protect the inner cells from the toxicity of high loadings, and the adsorption of contaminants within the biofilm reduces the soluble concentrations of contaminants

The fixed film reactor used for municipal sewage is the trickling filter reactor,with large packing, typically 50 mm diameter stones or synthetic plastic, anddownward liquid flow, producing a liquid film over the biofilm and leaving airvoidage within the packed bed.

Liquid hold-up is low and retention times are short, so that liquid recirculation is necessary to adequately reduce contaminant concentrations.

Recirculated effluent is usually taken from a secondary clarifier output, rather thandirectly from the trickling filter effluent, to minimize the risk of clogging thetrickling filter with biomass released from the filter.

Wastewater contaminated with xenobiotics is usually treated in a fixed filmreactor with upflow through a submerged plastic packing.

Nitrogen and phosphorus may be added to the inflow so that the C: N: P ratio is approximately 100: 5: 1.

Reactor cells may be staged, and typically a 2-hour residence time is sufficient to give a high level of contaminant degradation

The major advantage of the upflow fixed film reactor versus the trickling bed reactor is the longer and better control of residence time.

A good example of a successful application of a fixed film bioreactor is thetreatment of a lagoon contaminated with 36ppm pentachlorophenol (PCP),37 ppm polynuclear aromatics (PNAs), 52ppm solvents, and a total chemicaloxygen demand (COD) of 6700 ppm

Anaerobic processes

While anaerobic processes for bioremediation of groundwater are uncommon,anaerobic treatment is used for municipal sludge processing and for agriculturaland food-processing wastewaters with a high content of biodegradable matter.

The resultant methane production provides a valuable energy source.

Since energy production is not an option in the treatment of groundwater, anaerobic processes are considered only for compounds recalcitrant to aerobic degradation.

Anaerobic microorganisms have great potential for the reductive dehalogenation ofmultihalogenated aromatic compounds. With increasing levels of halogenation,bioremediation may be feasible only with an initial anaerobic dehalogenation.

Once lightly halogenated or nonhalogenated compounds are produced, subsequentdegradations are more rapid in an aerobic environment. This suggests ananaerobic-aerobic process for the degradation of compounds that are highlychlorinated or nitrated, or for PAHs.

The anaerobic bioreactor is typically an upflow packed-bed reactor. The reactor iscompletely filled with liquid except for gas formed during the process.

Coarse packing (2 to 6 cm) is used since anaerobic organisms can form large floes instead of thin attached films, causing clogging.

Anaerobic reactors have a number of advantages, including high efficiency at low organic loading, high loading capacity, stability with toxic substances, and low energy requirements. COD reductions of 4 to 10kg/m3/day can be obtained with residence times from 4 to 18 h.

A two-stage anaerobic-aerobic biofilm reactor process has been used tometabolize hexachlorobenzene (HCB), tetrachloroethylene (PCE), and chloroform(CF)

Reductive chlorination is relatively rapid for these compounds and other highly chlorinated compounds such as polychlorinated biphenyls, trichloroethylene, carbon tetrachloride and 1,1,1-trichloroethane.

These are some of the most pervasive groundwater contaminants.

2,4,6-Trichlorophenol can be degraded to 4-chlorophenol in the anaerobic reactor, and the 4-chlorophenol was mineralized in the subsequent aerobic operation.

A sequential anaerobic—aerobic process similar to the process shown in Figure 1.1 has been used to degrade nitrobenzene.

Under anaerobic conditions nitrobenzene was converted to aniline, a reaction accelerated by the addition of glucose. Complete mineralization of aniline was accomplished in the aerobic stage.

Anaerobic—aerobic processes have a high potential for the treatment of pulp millwastewater containing xenobiotic compounds.

The pulp and paper industry is under great pressure to remove chlorophenols, chlorinated aliphatic hydrocarbons, and chlorinated dioxins and furans from wastewater.

Soil remediation

With ex situ treatment of contaminated soils, a controlled environment for soiltreatment can be maintained.

With mixing, nutrient addition, aeration, and otherenvironmental controls, mass transfer rates that typically limit in situ bioremediationcan be greatly increased.

The disadvantages of ex situ bioremediation are the costs of soil excavation and reactor operation. Thus, ex situ bioremediation is favored by localized, shallow soil contamination.

Landfarming

Biological soil treatment by landfarming is a relatively simple and inexpensivemethod for treating soil contaminated by compounds that are readily degradedAerobically.

Contaminated soil is evacuated and usually treated in pits lined with a high-density synthetic or clay liner (Figure 1.2). Perforated pipes can be placed in a layer of sand between the liner and contaminated soil to collect drainage that can be separately treated or recycled.

Alternatively, the treatment area can be graded to a sump where runoff is collected. Aeration can be accomplished by tilling the soil or with forced aeration. With tillage, soil is usually spread to a depth of 15 to 50 cm. For forced aeration, soil is placed above slotted PVC pipes that are manifolded to a blower (Figure 1.2). Nutrients may be added and pH adjusted.

Phosphorus is typically added as a salt of phosphoric acid and nitrogen as anammonium salt, a nitrate salt, or urea. Nutrient requirements are estimated fromcontamination concentrations or laboratory treatability tests, and water is added orsprayed onto the soil to maintain optimum moisture.

Landfarming has been widely implemented at petroleum production and storagesites, and at sites contaminated with polynuclear aromatic residues (PNAs) orpentachlorophenol (PCP).

The efficacy and design of land treatment for petroleum-contaminated soil hasbeen studied in controlled laboratory experiments.

The effects of soil type, fuel type, contamination level, and temperature on the kinetics of fuel disappearance were determined for a land treatment process using lime to raise the pH to 7.5-7.6, addition of 60 µ mol of N as NH4NO3 and 5 µ mol of P as K2HPO4, and tilling

Disappearance of hydrocarbons was maximal at 27 °C. The C6 to C9 components of gasoline were removed primarily by evaporation. The C10 to C 11 components were removed by biodegradation. The medium distillates responded well to bioremediation and increased in persistence in this order: jet fuel, heating oil, and diesel oil.

Biodegradation can be accelerated in a prepared bed reactor with forced aeration.These reactors are used at many Superfund sites for bioremediation ofPAHs and BTEX (benzene, toluene, ethylbenzene, and xylene).

A potential problem in soil treatment is the residual contaminant concentrationthat is slowly or not noticeably degraded by soil microorganisms.

This nonbioavailable fraction is recognized by its slow transport out of soil micropores.

Slow desorption can result from entrapment in intraparticle micropores, especially in the presence of organic matter, which can tightly bind nonionic organiccontaminants.

Soil leaching experiments can demonstrate whether contaminants are slowly Released from the soil matrix, and the addition of surfactants can increase the Desorption rate.

However, the mixing and surfactant addition that may be necessary to releasecontaminants increase the cost of landfarming and favor the use of more intensivebioremediation methods such as slurry reactors.