Leachate Treatment Field Trials
This study, which was carried out in 1994, was intended to demonstrate the operational features of a pilot-scale anaerobic digester linked to a secondary stage nitrification plant for the on-site treatment of landfill leachate.
During the first year of operation, excessive inorganic scale accumulation on the interior of a 4m3 Upflow Anaerobic Sludge-Bed/Filter reactor and pipework surfaces resulted in operational problems, and forced the eventual shutdown of the pilot-plant. These studies highlighted the requirement for iron removal prior to anaerobic treatment, as evidenced by the low percentage of volatile matter (17.5%) and the high proportion of iron (10.5% w/w) in the sludge after operation for one year. Pre-treatment of the metals within the leachate was initiated by the addition of alkali, for metal precipitation, followed by neutralisation with acid. The maximum efficiency of the pre-treatment system was between 0.1 and 0.15 kgFe.m-3.d-1, which was approximately 60% of the maximum loading rate. Pre-treatment with alkali also removed calcium, zinc and phosphate to varying amounts. Results presented in this study suggested that a COD:phosphate ratio of 1000:1 was sufficient to support anaerobic decomposition within the digester, under the conditions used.
Extended operation of the pilot-scale digester at a HRT of 2 days resulted in a maximum treatment of 4 kgCOD.m-3.d-1, equating to 55% removal of COD. The relatively low organic treatment efficiency observed in comparison to previous laboratory studies, where treatment efficiencies in excess of 10 kgCOD.m-3.d-1 have been recorded, was shown to be caused by: (i) the variable leachate strength; (ii) heavy metal concentration leading to blockages; (iii) leachate feed arrangement to the reactor; and, (iv) poor hydraulic contact with the sludge.
The results of this study showed that the process of nitrification was inhibited at COD and BOD loadings above 0.5 and 0.25 kg.m-3.d-1, respectively. The study proved the necessity for a secondary stage aerobic treatment to metabolise residual organic material from the digester effluent. Treatment of residual organics in the digester effluent by the activated sludge unit provided an effluent of nutrient quality sufficient for nitrification. Subsequent studies showed that operation of the anaerobic digester, activated sludge unit and a Rotating Bological Contactor (RBC) in series proved successful as a treatment process for the removal of organics and the nitrification of ammonia from leachate.
This study further demonstrated the possibility of treating high concentrations of nitrate, resulting from the nitrification of leachate. The use of an external carbon source for denitrification was unnecessary. In terms of organic and nitrogen removal, high efficiencies were achieved. A four stage, organic-nitrification-denitrification system used in this study was capable of reducing COD, BOD and ammoniacal-nitrogen concentrations of 15,000, 9,240 and 1,100 mg.l-1 to 1,300, 35 and 45 mg.l-1, respectively.
The information supplied drew attention to those process features of the trial which have positively and negitavely influenced the process performance. The use of an anaerobic upflow sludge bed/filter and an attached-growth system for denitrification, such as a fluidised-bed reactor, were proposed as improvements to system used in this study.