Precipitation/ Coagulation/ Flocculation

Process Type: Chemico-physical

Description:

Chemical precipitation of metals

It has been widely demonstrated that, with the exception of levels of zinc in acetogenic leachate samples, concentrations of heavy metals in leachates from landfills containing primarily household wastes are relatively low. Typical values are generally lower than those measured in samples of domestic sewage, and far lower than levels of metals being treated at Sewage Treatment Works, where inputs of industrial effluent have also been received. Median values for key metals in leachates from modern large landfills, with high waste input rates (including co-disposal sites and sites receiving industrial and commercial wastes) are reported below, and are compared with values for non-industrial crude sewage (in mg/l, after Robinson, 1995).

Additionally, significant removal of some of these metals in leachate (e.g. zinc, chromium, copper) has been reported during aerobic biological treatment (see Robinson and Knox, 2001).

On this basis, chemical treatment to reduce concentrations of metals is unlikely to be widely required, in particular at landfills which, receive significant inputs of household wastes, or where leachates are treated biologically before discharge.

Precipitation and other reactions within an anaerobic landfill, will generally reduce the mobility of heavy metals significantly.

Nevertheless, if specific circumstances require such metal removal, chemical precipitation processes are widely employed for this purpose, for effluents in a wide variety of industries, and could readily be adopted.

Although specific treatment processes for removal of heavy metals from landfill leachates will only occasionally be necessary, and have rarely been provided at UK landfill sites, on occasions when specific features of landfills require such treatment, there is a wealth of experience and data to allow appropriate systems to be designed (e.g. see Eckenfelder, 1989).

Difficulties may arise from the relatively low concentrations of heavy metals present in leachates, reducing the cost-effectiveness of the process, and also where there is a need to remove mixtures of metals, and these have different optimum pH-values for precipitation.

Metals Precipitation Process Overview

Precipitation is widely employed for the removal of concentrations of heavy metals from industrial wastewaters, and although many chemicals have been used (e.g. hydrated lime, quicklime, magnesium hydroxide, sodium hydroxide), hydrated lime, Ca(OH)₂, has been most widely used, and is generally the cheapest. Heavy metals are usually precipitated as the hydroxide through the addition of alkali, to a pH-value at which solubility of the metal of interest is minimised. Several metals are amphoteric, and exhibit a point of minimum solubility, below or above which solubility will increase and removal will reduce. Examples are chromium (pH value 7.5), and zinc (pH value 10.2).

Although many leachates have been shown to contain organic complexing agents, which have potential to interfere with metal removal (especially at relatively low concentrations in leachate), excellent removal of metals has nevertheless been reported by many authors (e.g. Knox, 1983; Bjorkman and Mavinic, 1977; Chian and DeWalle, 1977).

All precipitation processes are very strongly influenced by the pollution matrix of specific leachates, and as a consequence, laboratory and pilot-scale trials are essential if the process is to be optimised, and efficient treatment systems are to be developed, and operated to achieve effluent limits reliable and cost-effectively.

The wastewater treatment industry has extensive experience which enables it to provide appropriate precipitation processes, which take advantage of a range of chemical phenomena including co-precipitation and adsorptive co-precipitation, so that residual metal solubility levels far below theoretical solubility limits for simple metal salts can commonly be achieved.

Similarly, appropriate subsequent treatment stages of flocculation, sedimentation and clarification, can be optimised based on experience. Volumes and handling characteristics of precipitated sludges are frequently at least as important as economic factors, in final selection or optimisation of precipitation processes.

Coagulation and Flocculation

Chemical coagulation and flocculation are used for the removal of waste materials present in suspended or colloidal form. Colloids represent particles typically within a size range from 1.0nm to 0.1nm (10 - 7 to 10 - 8 cm). These particles do not settle out on standing, and are not readily removed by conventional physical treatment processes.

Coagulants, usually salts of iron or aluminium, are added at controlled pH-values to form solid precipitates termed floc, which contain the colloidal particles, and can then be separated out using conventional solid, liquid separation processes. The process of flocculation encourages floc growth by gentle mixing, to suite the subsequent separation process being used.

Coagulation and Flocculation Process Overview

In leachate treatment at UK landfills, full-scale coagulation/flocculation systems have rarely, if ever, been applied to the raw leachates, and only occasionally to biologically pre-treated effluents.

Nevertheless, in other countries such as Germany, coagulation and flocculation processes are more widely applied to both raw and treated leachates, and extensive experience is available. Common applications have included:

Removal of turbidity and colour from biological treatment effluents;
Reduction in COD values associated with colloidal materials;
Removal of powdered activated carbon (PAC) in effluent polishing separate section);

Reduction in suspended solids concentrations, to protect subsequent treatment stages e.g. in activated carbon columns.

Coagulant aids, often polyelectrolyte compounds, may be added to enhance coagulation by promoting the development of large, rapid-setting flocs.

Polyelectrolytes are high-molecular-weight polymers that form bridges between particles or charged flocs, when added at low concentrations (1-5 mg/l) in conjunction with alum or ferric chloride.

The key to successful coagulation and flocculation is detailed jar-scale, laboratory testing, to establish the optimum pH-value and coagulant dosing for treatment of a specific leachate or effluent. Good mixing at the point of chemical dosing, and tight control of coagulant dose and pH-value are essential, as is optimisation of the physical process of floc formation. In large-scale wastewater treatment processes, sophisticated feedback controls are routinely used, which may be more difficult to apply to smaller leachate treatment applications.

Environmental issues and Concerns

Principle environmental issues relate to the correct storage of chemicals, correct dosing to prevent excessive use of reagents and sludge disposal. Sludge may be dewatered to facilitate handling transportation and disposal. Typically, disposal will be by landfill depending on Landfill Regulations limitations.

Advantages:

Comparably low capital cost.

Disadvantages:

Controllability may be difficult if the leachate strength fluctuates.

Where best used:

Instances where biological treatment cannot be used.

Costs comments:

Sustainability comments:

Principle environmental issues relate to the correct storage of chemicals, correct dosing to prevent excessive use of reagents and sludge disposal. Sludge may be dewatered to facilitate handling transportation and disposal. Typically, disposal will by landfill depending on Landfill Regulations limitations.