Question

A slurry-phase treatment is applied to remove organic pollutants from contaminated soils. A pretreatment is first applied to screen out gravel and sand from the soil, and to clean these fractions with water. On the contrary, the remediation of silt and clay fractions of the soil requires an addition of surfactant prior to biological treatment. Please explain the rationale of the treatments. (around 400 words in details explanation)

Answer 1

Bioremediation is an alternative that offers the possibility to destroy toxic pollutant using natural biological activity. By definition, bioremediation is the use of living organisms, primarily microorganisms, to degrade the environmental contaminants into less toxic forms. It uses naturally occurring bacteria and fungi or plants to degrade or detoxify substances hazardous to human health and/or the environment

Bioremediation techniques are destruction or transformation techniques directed toward stimulating the microorganisms to grow and use the contaminants as a food and energy source by creating a favorable environment for the microorganisms. Generally, this means providing some combination of oxygen, nutrients, and moisture, and controlling the temperature and pH. Sometimes, microorganisms adapted for degradation of the specific contaminants are applied to enhance the process.

Bioremediation processes have been classified in three broad categories, according to place and soil handling/conditioning: in situ, ad situ, and ex situ. The second and third class of processes are useful for the remediation of sludges, soils or sediments polluted with high concentration of recalcitrant contaminants , for instance polynuclear aromatic hydrocarbons , diesel , explosives , pesticides and chlorinated organic pollutants, oily sludges from the petrochemical industry ; clayish and stratified soils with low hydraulic conductivity and low permeability accompanied with high contents of organic matter

Process .....

In slurry-phase treatment , soil is excavated and conditioned and loaded into bioreactors . A main feature of slurry-phase treatment is that soil inside reactor is kept in aqueous suspension by some type of mixing in a way that biological treatment is carried out under saturated conditions and nearly homogeneous suspension.

There are several bioreactor configurations . At full scale, low cost bioreactors may consist of large lined lagoons (24 m × 15 m). Manufactured bioreactors can range 3 to 25 m diameter and 4.5 to 8 m height, with capacities between 60 to 1000 m3 . Bioreactors are usually fitted with mixing devices, and aerobic slurry-phase treatment are equipped with spargers or diffusers. Ancillary equipment may include gas emissions conduits and treatment, vessels for nutrient and pH conditioning of slurry, etc.

Main operating modes of slurry-phase treatment include batch and semi-continuous. Continuous slurry-phase treatment are possible in principle, but are not very common. Batch and semi-continuous reactors adapt easily to the handling of soils and slurries , sometimes are operated in sequencing batch reactors

Soil pretreatment.....

As it was mentioned above, one of the limitations of slurry-phase treatment technology is the requirement of soil excavation and pre-treatment. Usually, pretreatment consists of crushing followed by screening. The coarser fractions of soils (pebbles and sands, 0.85 to 4 mm) are discarded and sent to direct disposal, whereas fine fractions are retained and loaded into bioreactors. It is generally recognized that pollutants concentrate in fine particles of soil .

Solids concentration....

Polluted, fine fractions of soils are mixed with water or wastewater to form slurry with a concentration in the range between 15 to 60% w/v, depending on characteristics of soil and degradation rates determined in previous laboratory or pilot scale studies . Solids concentration is a key variable that might determine the mixing power required, aeration efficiency in aerobic slurry-phase treatment, and the size of by-stream post-treatment installations . It is a common practice to use decanted spent liquors from previous batch runs as water for making the slurries, as a method to minimize liquor treatment and disposal.

Mixing....

SBs are provided with mixing devices in order to keep solids in suspension during biotreatment, as well as intensifying turbulence and thus increasing mass transfer rates . There are several types of mixing devices, mechanical and pneumatic ones are the predominant among aerobic slurry-phase treatment. Also, mixing can be intermittent or continuous. The first leads to significant savings on power expenses, although mixing intensity is lower.

Mixing intensity is a critical factor in slurry-phase treatment design and performance . Its main role is to keep solid particles in suspension and provide slurry homogeneization, to help achieving a satisfactory aeration in aerobic SBs, and to increase several mass transfer rates such as desorption of pollutants from soil, among others. This, in turn, usually translates in higher rates of contaminant biodegradation, in particular when difficult-to-degrade xenobiotic compounds such as PCBs have to be removed . Mixing selection and sizing mainly depends on slurry characteristics and kinetic requirements. Pilot studies and semi-empirical methods are usually required. Independently of the type of mixer selected, mixing power is of major concern because of its influence on the operating costs. The denser the slurry, the higher the power and more difficult to achieve oxygen transfer in aerobic slurry-phase treatment. So, several trade-offs have been recommended to keep power expenses at bay, such as intermittent mixing and slurry dilution.

Control of environmental conditions in the operation of slurry bioreactors....

One distinctive and advantageous feature of SBs is the manipulation and control of environmental conditions that leads to biodegradation optimization and better process performance . In this regard, several operational variables can be monitored and controlled: pH, dissolved oxygen in aerobic slurry-phase treatment, concentration of inorganic nutrients, pH is usually kept in the range 6.75–7.25 using either alkalis such as NaOH or acids such as H2SO4. Temperature range is 25–30°C, although SBs have been reported to successfully perform at lower ambient temperatures. Ideal dissolved oxygen concentration is 90% of the saturated dissolved concentration. Nitrogen and phosphorus salts are used as inorganic nutrient sources in order to ensure that there is no nutrient limitation (NH4Cl, KH2PO4, Na2HPO4)