Report No DWI0202

Nov 1978

Exposure to excessive concentrations of cadmium and mercury has produced adverse effects on health in certain communities. Fears exist that some of the other so called "heavy metals" may have similar toxic effects. In addition,some of these elements may have detrimental effects upon crop growth and yield. They may also have adverse effects in the environment on other living organisms if their disposal and dispersal is not regulated in such a manner as to prevent the accumulation of toxic concentrations.

Raw sewage contains significant concentrations of "heavy metals" Which are not degraded by the processes of sewage treatment, they may leave the sewage treatment works in either the final effluent or the sludges produced. Monitoring the concentrations of these metals in the various types of samples produced during sewage treatment is a time consuming task. It is necessary if guidelines for the disposal of sewage sludge to agricultural land are to be complied with, and if rivers to which sewage effluent is discharged,and which are subsequently used as sources of potable supply, are to be protected from excessive concentrations of certain heavy metals, which might infringe standard for drinking water quality.

Colorimetric methods are currently recommended for the analysis of aluminium, calcium, cadmium, copper, chromium, iron, magnesium, molybdenum, nickel, lead and zinc in sewage and sewage sludges in the United Kingdom. In recent years flame atomic absorption spectrophotometry has been extensively used for the analysis of metals in many types of samples. Both colorimetry and atomic absorption spectrophotmetry require the destruction of any organic matter present in the sample prior to analysis.

While a variety of methods exist for the destruction of organic matter, most are time consuming and few are appropriate pretreatments for all of the common determinands. Flameless atomic absorption spectrophotometry using an electrothermal atomiser offers the opportunity to avoid these time consuming pretreatment procedures. The programmable nature of the furnace power supply permits control of the temperature and time in the drying, ashing and atomisation stages of the analytical cycle. Selection of the appropriate conditions for the ashing stage and the use of deuterium background correction to avoid "non atomic" interferences eliminates the need for complex and time consuming pretreatment. This flameless atomic absorption technique,in conjunction with sample pretreatment by homogenisation (to ensure that the solids are uniformly dispersed permitting representative sub-sampling),has been evaluated by comparison with standard and recommended methods of analysis.

Results presented have been statistically analysed and indicate that the rapid flameless atomic absorption procedure is suitable for the analysis of aluminium, calcium, cadmium, chromium, copper, iron, magnesium, molybdenum, nickel, lead and zinc in sewage sludge, sewages and final effluent. A digestion procedure based upon the use of hydrogen peroxide and nitric acid in conjunction with flame atomic absorption analysis gave results comparable to those of the rapid flameless atomic absorption method for many of these determinands in most types of samples. It would appear that pretreatment by ashing prior to acid dissolution and flame atomic absorption analysis may be of limited value for the analysis of sewage sludge. The merit of the flameless atomic absorption technique and its value for the analysis of sewage sludges, sewages and effluent is discussed.

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