Management Effects And Their Costs Of Land Treated Meat-Processing Wastewater On Groundwater Quality

ABSTRACT - Rob Potts

A land treatment system is a combination of the following treatment systems: air stripping for ammonia volatilisation; ion adsorption and exchange system for adsorbing cations; fixed media trickling filter for both physical filtering and biochemical degradation; biological nutrient reduction system via plant removal, microbe use and denitrification; and disposal system to groundwater. Richmond Limited at Takapau have been monitoring their effects on groundwater in their vicinity for the last 17 years. Past nitrogen loading onto border dyke land treatment areas has exceeded 5,000 kg/ha/year, resulting in groundwater nitrate nitrogen levels reaching 80 g/m3. Major changes to the land treatment system have been introduced over the last five years, including; an increase in land treatment area, change to spray irrigation, and change in irrigation management of poorer soil types. These changes have reduced the groundwater nitrate nitrogen concentrations from 80 g/m3 to 20 - 30 g/m3, and significantly reduced the odour complaints by neighbours. The capital cost of these changes to the company were in the order of $800,000, not including land costs. A mass balance modelling approach predicted worst case downgradient groundwater nitrate levels for proposed future management changes to less than 7 g/m3 above background levels. Future management changes should result in an odourless environment for neighbours, and gradually improving groundwater quality. The capital costs to implement these changes are minor but require a change in processing plant and irrigation system operation. In addition, the results from the environmental monitoring were used to calibrate the mass balance model. This showed that groundwater characteristics that had historically been used for groundwater modelling in the area were significantly different to the characteristics at the site and grossly underestimated the degree of contamination that would move off the site.