The biodegradation of organic chemical waste using a constructed horizontal reedbed treatment system

Abstract

The aim of the project was to evaluate the potential for a constructed reedbed (planted with Phragmites australis) to aid in the biodegradation of triethylene glycol (TEG) under normal climatic conditions experienced at the site. The approach taken was to design and construct a series of test beds which could be used in replicated experiments to measure the rate at which TEG solutions of different concentrations were degraded. The effect of the TEG on reeds and the interactions between TEG and physical conditions within the test beds were monitored and in addition, the potential use of two methods which might be used instead of the BOD(_5) method were assessed.Although the temperature range experienced during the series ranged from 2.5 C to18.5 C, TEG was degraded within four days at both high and low temperatures. Theinitial decline in TEG concentrations was rapid and this was thought to be partly due tosome dilution by water but mainly by the action of bacteria within the reedbed.Subsequently, TEG concentration declined more slowly. The TEG concentrations used inthe tests were between 0.1 %v/v and 5.0%v/v with the higher concentrations degradingfaster initially, but the overall degradation rates being similar. The physical conditionswithin the treatment tanks compared to the controls showed that temperature was notaffected, pH was unaffected in the early trials but showed slight changes to aciditytowards the final trials. Conductivity in both control and treatment beds showed similarunexplainable variances until the later treatments when the treatment beds displayed higher values than those found in the control beds. Dissolved oxygen in the treatment beds was lower than in the untreated beds as was expected due to bacterial action. The effect of dosing with TEG on the reeds was to increase the number of shoots per area and Total Kjeldhal Nitrogen (TKN) content after one season with biomass production of the treated beds increasing after two years. The stomatal count on second year plants showed a significant increase in the treated reeds against the untreated ones. A brief investigation of the impact of TEG on micro-organisms within the test beds indicated that some species of bacteria were probably 'tegophilic' i.e. bacteria that flourish in a medium containing TEG and that сіliate protozoa were not adversely affected. Of the two methods examined to replace the BOD(_5) test, the EZBOD® meter was found unsuitable due, it was thought, to the bactericidal properties of the TEG. Trials using a Total Organic Carbon analyser indicated that this rapid method would be a successful supplement and/or replacement to the BOD(_5) method currently used to monitor whether waste water quality meets disposal requirements imposed by the Environment Agency. It was found that the TEG did biodegrade without having any adverse effects on the reeds and that the degradation was a first order reaction. A reaction rate was determined that will enable anybody to determine the requirements of a treatment system to deal with effluent containing TEG.