The wine industry produces large quantities of wastewater associated with washing operations during grape harvesting, pressing and first fermentation phases of wine processing (Mulidzi, 2007). Traditionally wastewaters from wineries and distilleries have been disposed of in evaporation ponds and in some cases in natural water courses (Shepherd, 2002). In South Africa more than 60% of wineries still dispose their effluent by means of land application (Mulidzi, 2008). Constructed wetlands have been investigated extensively in the last twenty years for wastewater treatments. The motivation for using constructed wetlands for treating winery wastewater is that wetlands are naturally among the most biologically active ecosystems on earth (Cooper, 1999). Constructed wetlands are created for specific purposes, in this case distillery and winery wastewater treatment (Van Schoor, 2002). Furthermore, wetlands are among the least expensive treatment systems as compared to other wastewater treatment systems. For the past 10 years, research trials on the use of constructed wetland to treat winery and distillery wastewater were conducted in Goudini near Worcester in Western Cape. The results had shown that constructed wetlands with retention time of 14 days were effective in treating COD from the wastewater.
The aim of the study was to investigate the effect of seven days’ retention time on the performance of constructed wetlands. The study was conducted at the same wetland that was used for previous studies, i.e. 14 days retention time. The reason was to enable the research team to compare the previous results of 14 days retention time with the new results of seven days retention time.
Material and methods
The wetland was 45 m long, 4 m wide and 1.2 m deep. The wetland was lined with a 1 mm thick dam liner and filled to a depth of 0.9 m with dolomitic gravel having a particle size range of 20 to 30 mm. The porosity of the gravel bed was 35%, giving a total wetland volume of 162 m3 and a pore volume (accessible to roots and wastewater) of 56.7 m3.
The following wetland plants were used for the trial: Typha, bulrush and cattails were planted at a spacing rate of 10 plants/m2. Mulidzi (2005) presented a partial list to assist in plant selection (Table 1). The capacity of the wastewater treatment system to work depends on how long the water stays in the system. The retention time of the constructed wetland was seven days (8 100 ℓ/day). Wastewater was sampled prior to wetland and after the wetland on a weekly basis in order to determine the performance of the system. The wastewater was analysed for COD, pH, potassium, nitrogen, conductivity, calcium, sodium, magnesium and boron.
Results and discussion
Evaluation of water quality after treatment
The application of wastewater with a retention time of seven days (8 100 ℓ/day), as well as the evaluation of water quality after treatment was carried out the whole year. The results had shown an overall average COD removal of 60% throughout the year. The results showed low COD removal during July until September after which it improved tremendously (Figure 1). The reason for low COD removal during the first three months could be attributed to the fact that there was no gradual application of wastewater to the wetlands, i.e. from 4 050 ℓ/day to 8 100 ℓ/day. This sharp increase of volume could have an adverse effect on the composition and performance of micro-organisms in the system. Under normal circumstances, there should be a gradual increase of volume, but this was done deliberately in order to find out how the system would react to an abrupt change in flow and volume.
FIGURE 1. COD results for Goudini constructed wetland with seven days retention time (8 100 ℓ/day) indicating an average overall percentage COD removal of 60%.
Monitoring and maintenance of constructed wetlands
When the wetland plants were more than 1 m, they were harvested in strips of 2 m starting from the inlet according to procedures developed from previous seasons (Mulidzi, 2007). The idea behind harvesting plants in strips was that when you finish harvesting the whole wetland, plants at the beginning will be ready for harvest again. These stimulate plant growth even during winter periods, thereby improving wetland performance. Due to vigorous growth of wetland plants, routine removal of excess plants, as well as dead materials in the wetland system is recommended to avoid clogging of the system.
A wetland is a biological filter and like all filters, requires some occasional cleaning and maintenance (Kadlec, 1997). A big advantage of a wetland system is that less maintenance is needed compared to other treatment systems. Routine removal of excess plant material and captured sediments will allow the wetland to continue to function. Regular harvesting of the crop can prevent the system from becoming clogged.
Conclusions and recommendations
The results had showed that constructed wetland as a secondary treatment system is effective in terms of COD and other elements removal from winery and distillery wastewater. COD removal throughout the year was 60% with seven days retention time. When compared with previous studies of 80% COD removal with 14 days retention time, the 60% is very critical to wine industries as more wastewater is applied to the system. Although the retention time of seven days did not get the same water quality compared to 14 days, it is very important to know the level of performance by wetlands at a shorter retention time as it will serve as guideline for future consultations and as a basis for future research. The retention time plays an important role in terms of the size of wetland to be constructed. It is recommended that a compulsory pre-treatment system for solids removal is crucial, as solids contains more than 40% of the COD load and the solids can cause clogging of the system if not removed.
The rationale for using constructed wetlands for treating wastewater is that wetlands are naturally among the most biologically active ecosystems on earth. The aim of the study was to determine the impact of shorter retention time on the performance of constructed wetland in terms of chemical oxygen demand (COD) and other elements removal. The application of wastewater with retention time of seven days, as well as the evaluation of water quality after treatment by constructed wetland was carried out throughout the year. The results had shown an overall average COD removal of 60% throughout the year.
Cooper, P., 1999. A review of the design and performance of vertical flow and hybrid reed bed treatment systems. Water Science and Technology 40(3), 1 – 9.
Kadlec, R.H., 1997. Deterministic and stochastic aspects of constructed wetlands performance and design. Water Science and Technology 35(5), 149 – 156.
Mulidzi, A.R., 2007. Winery wastewater treatment by constructed wetlands and the use of treated wastewater for cash crop production. Water Science and Technology 56(2), 103 – 109.
Mulidzi, A.R., 2008. Cost of constructed wetland at Goudini Distillery. WineLand, February, 66 – 67.
Mulidzi, A.R., 2005. Monitoring performance of constructed wetlands in California. WineLand, May, 85 – 87.
Shepherd, H.L., 2002. Use of constructed wetlands in treating winery process wastewater: Design issues and field investigations. International Workshop and Seminar on “Effects and treatment of cellar and distillery effluent”, Stellenbosch, South Africa, 23 – 24 April.
Van Schoor, L.H., 2002. The use of artificial wetlands in the purification of winery wastewater. WineLand, March, 33 – 35.
This article originates from research funded by Winetech and the final report of project WW 19-13, “The impact of shorter retention time on wastewater treatment by constructed wetlands”, can be downloaded fromhttp://www.sawislibrary.co.za/dbtextimages/Winetech2010_21.pdf.
– For more information, contact Reckson Mulidzi at MulidziR@arc.agric.za.