THE EFFECT OF EBB-AND-FLOW TECHNOLOGY, SUBSTRATE TYPES AND SALT-TOLERANT CROP (Apium graveolens L.) ON NUTRIENT REMOVAL FROM A BREWERY EFFLUENT

Abstract

Brewing industries face economic and environmental challenges of water use, energy consumption, and effluent disposal. The limited water availability and strict effluent discharge regulations in the South African Brewery justify wastewater treatment and recycling. Constructed wetland (CW) design and operation have been optimised for sustainable wastewater treatment. An ebb-and-flow technology is a CW design that creates aerobic and anaerobic conditions essential for wastewater treatment. South African Ibhayi Brewery effluent is treated onsite using a commercial-scale anaerobic digester (AD) and activated sludge units. However, the anaerobically digested effluent has high treatment costs and does not meet the environmental discharge standards. Therefore, the post-AD effluent is further treated using a low-cost CW technology. This study investigated the effect of ebb-and-flow CW operation, substrate types and celery plant growth on nutrient removal from a post-AD brewery effluent. Two experiments were conducted; Experiment 1 examined the effect of retention times (10, 20 and 40 minutes) and gravel sizes (7, 13, and 19 mm) on nutrient removal. The retention times (RT) and gravel sizes were allocated as treatment combinations and replicated thrice in a completely randomised design. In the second experiment, an ebb-and-flow CW was used to investigate the effect of media types planted with a celery crop on nutrient removal. Eight media (clay pebbles, clay bricks, sand, bioballs, recycled plastic, and gravel sizes), either alone or mixed, were tested. A 2:1:1 ratio of each media, pine bark and granular activated carbon, respectively, was used for mixed treatments. A uniform celery planting density of eight seedlings per m 2 and a 12-minute effluent retention time were used. Results for experiment 1 indicated significant differences at p≤ 0.05 between RT and gravel size treatment combinations on nutrient removal. The 10-minute RT at 19-mm gravel achieved the highest reduction in chemical oxygen demand (COD) of 8.2 %, 7.7 % ammonia-N and 38 % total inorganic nitrogen removal. The 40-minute RT at 7-mm gravel had the highest nitrate-N removal of 18.6 %. Orthophosphate removal was below 5 % in all treatments. The peak removal for ammonia and total inorganic nitrogen was after 8 and 10 weeks, respectively, suggesting that the efficiency of the ebb-and-flow design on nutrient removal is time-dependent. The aerobic and anaerobic conditions of ebb-and-flow operation, gravel surface area and effluent retention time influence nutrient transformation and removal. Results for Experiment 2 indicated significant differences between unmixed and mixed media on nutrient removal (p ≤0.05). The unmixed media of clay pebbles had the highest mean reduction chemical oxygen demand of 7.5% and 8.1% ammonia-N removal. Mixing resulted in better overall mean removal efficiency of nitrite- N (7.6%), nitrate-N (15.3%), total inorganic nitrogen (29.9%), orthophosphate (12.7%) and plant biomass (11158.5 gm -2 ) than unmixed media. Media porosity, surface area, chemical composition and celery plant growth enhanced nutrient removal through synergistic interactions. Experiment 1 recommends a short effluent retention time of 10 minutes on 19-mm gravel for improved nutrient removal in the ebb-and-flow system design. Experiment 2 recommends unmixed clay pebbles, bioballs, and 19-mm gravel forimproved ammonia-N removal and mixed media for multiple pollutant removal and celery productivity in the ebb-and-flow constructed wetland.