Many treatment methods used to remove some of specific compounds or groups of compounds are given below:
Three of the processes commonly used are activated-carbon adsorption, activated-sludge, powdered activated carbon and chemical oxidation. In process selection, pilot plant testing is recommended for the development of treatment performance data and design criteria.
1. Carbon Adsorption:
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Carbon adsorption is an advanced wastewater treatment method used for the removal of the refractory organic compounds as well as residual amounts of inorganic compounds such as nitrogen, sulphides, and heavy metals. Granular-medium filters are commonly used upstream of the activated-carbon contactors to remove the organics associated with the suspended solids present in secondary effluent.
High influent suspended-solids concentrations (more than 20 mg/l) will form deposits on the carbon granules resulting in pressure loss, flow channeling or blockages, and loss of adsorption capacity. If soluble organic removal is not maintained at a low level, more frequent regeneration of the carbon may be required. Lack of consistency in pH, temperature, and flow-rate may also affect performance of carbon contractors.
Both granular and powdered carbons are used and appear to have a low adsorption affinity for low molecular weight polar organic species. If biological activity is low in the carbon contactor or in other biological unit processes, these species are difficult to remove with activated carbon. Under normal conditions, after treatment with carbon, the effluent BOD ranges from 2 to 7 mg/l, and the effluent COD ranges from 10 to 20 mg/l. Under optimum conditions, it appears that the effluent COD can be reduced to about 10 mg/l.
Types of Carbon Contractors:
Several types of activated-carbon contractors are used for advanced wastewater treatment. Typical systems may be either pressure or gravity type and may be upflow-countercurrent type with packed or expanded carbon beds or may be used as upflow or downflow fixed- bed units with two or three columns in series.
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i. Upflow Columns:
Upflow columns are arranged so that the liquid moves from the base of the column upward. As the carbon absorbs organics, the apparent density of the carbon particles increases and encourages migration of the heavier or more spent carbon downward.
Upflow columns may have more carbon fines in the effluent than downflow columns because upflow tends to expand, not compress, the carbon. Bed expansion creates fines (because the carbon particles collide) and allows the fines to escape through passageways created by the expanded bed.
ii. Downflow Columns:
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Downflow columns usually consist of two or three columns operated in series. The advantage of a downflow design is that adsorption of organics and filtration of suspended solids is accomplished in a single step.
Downflow filters may require more frequent backwashing because of the accumulation of suspended material on the surface of the contactor. Plugging of the carbon pores may require premature removal of the carbon for regeneration, thereby decreasing the useful life of the carbon. Sand and gravel resting on a filter block form the supporting media for downflow contractors.
iii. Fixed Beds:
In fixed-bed contractors, the carbon remains fixed as in the down-flow mode. Fixed beds remove particulates and require backwashing to dispose of the accumulated particulate matter. Usually fixed beds employ downward flow to lessen the chance of accumulating particulate material in the bottom of the bed where the particulate material would be difficult to remove by backwashing.
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iv. Expanded Beds:
The upflow column led to the development of the moving- or pulsed bed. In this method, wastewater flows upward through a descending fixed bed of carbon. When the adsorptive capacity of the carbon at the bottom of the carbon is exhausted, the bottom portion of carbon is removed, and an equivalent amount of regenerated or virgin carbon is added to the top of the column. Because this type of contractor cannot be backwashed, residual organic content in the contactor influent should be very low to avoid plugging.
Sizing of Carbon Contactors:
The sizing of carbon contactors is based on four factors – (i) contact time; (ii) hydraulic loading rate; (iii) carbon depth; and (iv) the number of contactors. A minimum of two parallel carbon contactors is recommended for design. Multiple units permit one or more units to remain in operation while one unit is taken out of service for removal and regeneration of spent carbon or for maintenance.
2. Activated-Sludge-Powdered Activated-Carbon Treatment:
In this process, when the activated carbon is added directly to the aeration tank, biological oxidation and physical adsorption occur simultaneously. A feature of this process is that it can be integrated into existing activated-sludge systems at nominal capital cost.
The addition of powdered activated carbon has several process advantages including- (i) system stability during shock loads; (ii) reduction of refractory priority pollutants; (iii) colour and ammonia removal; and (iv) improved sludge settle-ability. In some industrial waste applications where nitrification is inhibited by toxic organics, the application of powdered activated carbon may reduce or limit this inhibition.
With higher sludge ages, the organic removal per unit of carbon is enhanced, thereby improving the process efficiency. Reasons cited for this phenomenon includes- (i) additional biodegradation due to decreased toxicity; (ii) degradation of normally non-degradable substances due to increased exposure time to the biomass through adsorption on the carbon; and (iii) replacement of low molecular weight compounds with high molecular weight compounds, resulting in improved adsorption efficiency and lower toxicity.
3. Chemical Oxidation:
In advanced wastewater treatment applications, chemical oxidation can be used to remove ammonia, to reduce the concentration of residual organics, and to reduce the bacterial and viral content of wastewaters. Because chlorine forms trihalomethanes (THMs) when added to wastewater, alternatives to chlorine have been investigated where THMs are of principal concern in drinking water supplies.
Alternative oxidants include chlorine dioxide and ozone. When these chemicals are used for this purpose, disinfection of the wastewater is usually an added benefit. A further benefit of using ozone is the removal of colour.
Typical chemical dosages for both chlorine and ozone for the oxidation of the organics in wastewater are as given below:
The dosages increase with the degree of treatment, which is reasonable when it is considered that the organic compounds that remain after biological treatment are typically composed of low molecular weight polar organic compounds and complex organic compounds built around the benzene ring structure.