LANDFILL sites earmarked for toxic waste disposal cannot be treated like just any other landfill. The waste being disposed needs to be carefully handled. And once dumped, measures to decontaminate the site should be taken (Water Report, Vol 5, No 3).
Sridhar Susarla, a researcher at the ecological chemistry and microbiology division of the National Institute for Resources ~nd Environment, Tsukuba, Japan, studied an improved version of toxic waste landfill in New Plymouth, New Zealand. The problem of contari1i- nation in this particular case arose from the fact that chemicals from the waste deposited by Ivon- Watkins Dow limited -producing phenoxy herbicides ( thosewhich contain organic compounds of the phenoxy group) and other The top layer prevents the entry of rain water; industrial chemicals - the bottom layer contains the chemicals within leached into the groundand surface water bodies after being buried for more than seven years. There were three such locations where seepage ofthis kind had occurred.
To contain the prob- lem, the company called in environmental consultants to study the situation and recommend possible solu- tions. A secure toxic waste landfill in New Zealand was constructed by D' Appolonia Engineers Incorporated of the USA, specialists in chemical waste disposal. The designers decided to adopt a plan that would contain the waste on the site itself rather than trartsporting it to another site.
The adsorption and desorption characteristics of phenoxyacetic acids (an organic chemical used in the pharmaceutical, pesti- cide and dye industry) and.chlorophenols (a substi- tuted phenol that exists in three isomeric forms - ortho, para and meta; all three are used for themanufacture of dyes.) contained in the landfill led Susarla to the conclusion that the chemicals could be treated by usinR water. By passing water at a certain temperature, having a certainPH, all the chemicals could be leachedcompletely. In the present case, thechemicals in the landfill were all watersoluble to varying degrees and couldtherefore be removed.
A landfill measuring 60 m X 40 in x9.5 in in area was constructed. It's mainfeatures were a double, lining consistingof a synthetic material (HDPE, or highdensity polymer) and low permeabilitysoil (clay) encompassing the waste; twoleachate collection systems sandwichingthe synthetic layer where the lower is aback-up in the case of failure of the synthetic layer, and a groundwater sampling system. Of the two leachate, systems, the primary one has eight 100 mm,diameter HDPE pipes*ith perforationson the lower half of the pipe, the totalpipe area provided is 1,500 sq in. Themain function of these leachate systemsis the distribution of water throughoutthe contaminated soil for c flection ofchemicals and further treatment. On thewhole, both- the top and the bottomlayers are designed specially to make thelandfill as secure and leak-proof as possible. This landfill design has a lifespanof 25 years by which time degradation ofthe chemicals is almost complete.According to Susairla, a more securelandfill site solved the immediate environmental problem, but the chemicals,remained for a long time.
Investigations were also conductedon the possibility of biological degradation of the chemicals. For the purpose, amixed natural microbial population ofPsuedomonas .- a fungal species - wasobtained, from Soil inoculum. Theresults wCre positive because both the'chemicals (phenoxyacetic acid andchlorophenol) degrade in the presenceof the culture. In the aerobic processwhich was used at the site, there was a92 per cent reduction in the toxicity ofthe leachate. The results of the activatedsludge system - which was tried outin the laboratory - showed lowconcentration& of residual organic andinorganic ions, rendering the leachatenon-hazardous. But, the best remediation method for leachates, both'environmentally and economically,is the activated sludge system followedby activated carbon absorption treatment. once the concentration ofchemicals reaches the regulatory limit, itis legally safe for, the industry toabandon the site.