Approximately 3 to 5 m3 of poor quality wastewater is produced per tonne of grapes crushed by wineries (Mosse et al., 2011). Winery wastewater contains high levels of potassium (K) and sodium (Na) which originate from cleaning products, grape lees and spillage from the grape fermentation process (Laurenson et al., 2012; Conradie et al., 2014). Where wineries are surrounded by vineyards, irrigation with winery wastewater could be an alternative to fresh water abstracted from natural resources. However, winemakers are generally reluctant to dispose of winery wastewater for vineyard irrigation due to high Na and K (Kumar et al., 2014).
Potassium is the predominant cation involved in the pH balance of grape juice or wine, and there is a good relationship between pH and K (Kodur, 2011 and references therein). During winemaking, high wine K increases the precipitation of tartaric acid, consequently reducing free tartaric acid (Kodur, 2011). High wine K makes pH adjustment difficult and expensive (Kumar et al., 2014). High juice K can also reduce the tartaric/malic acid ratio which is undesirable for high wine quality (Mpelasoka et al., 2003). Elevated berry K will influence the effect of other cations (Boulton, 1980; Kumar et al., 2014) and is thought to have a negative impact on fermentation and microbial activity, as well as wine properties such as taste, bitterness and sourness.
According to Jackson and Lombard (1993), high levels of K in the juice are not only associated with high pH, but also poor colour of red wines. Although high K levels in the plant are correlated with high soil K concentrations, soil K effects on juice levels are small unless excessive K is applied. Although application of wastewater with high K levels will increase soil fertility, long-term application may cause accumulation of soil K (Kumar et al., 2014). This could decrease soil hydraulic conductivity (Arienzo et al., 2009). Currently, the Department of Water and Sanitation is drafting new General Authorisations for winery wastewater. The objective of this study was to determine the effect of using diluted winery wastewater for vineyard irrigation on K, Na and pH in juice and wine in order to make recommendations for possible adaptations to the General Authorisations.
The field experiment was carried out with micro-sprinkler irrigated Cabernet Sauvignon/99R in a sandy soil near Rawsonville in the Breede River Valley region from 2009/10 until 2012/13. Irrigation of grapevines using winery wastewater diluted to 100, 250, 500, 1 000, 1 500, 2 000, 2 500 and 3 000 mg/ℓ chemical oxygen demand (COD) was compared to a control of irrigation using river water. A cover crop was cultivated in summer to intercept salts applied via the winery wastewater. More complete details on the wine preparation and analyses methods have been published by the authors (Myburgh & Howell, 2014).
Results and discussion
In general, juice K and Na were within the recommended levels (Wooldridge et al., 2010). Juice K tended to increase with an increase in COD level of the diluted winery wastewater (Table 1). Juice K also tended to increase where undiluted winery wastewater was used for vineyard irrigation (Kumar et al., 2014). It has also been reported that irrigation using artificial winery wastewater containing wine and high K levels produced juice with lower K compared to wastewater containing high K and Na (Mosse et al., 2013). This indicated that the presence of wine in the artificial winery wastewater prevented an increase in juice K. In another study, juice K increased where 45 kg/ha K was applied as vineyard fertiliser compared to where no K was applied (Conradie & Saayman, 1989). However, there were no further increases in juice K when the fertiliser application was 90 kg/ha. Another study indicated that when 450 kg/ha K was applied to grapevines, there was an increase in juice K (Morris & Cawthon, 1982). Juice K of Concord grapevines also increased when K application increased from none, to 225, 450 or 900 kg/ha K (Morris et al., 1980).
TABLE 1. Effect of irrigation using winery wastewater diluted to different levels of COD on juice and wine K, Na and pH of Cabernet Sauvignon/99R near Rawsonville. Data are means of three seasons.
|Measured variable||River water (control)||Level of COD(1) in diluted winery wastewater (mg/ℓ)|
|100||250||500||1 000||1 500||2 000||2 500||3 000|
|Juice K (mg/ℓ)||1 814||1 931||1 856||2 020||2 126||2 158||2 226||2 158||2 345|
|Juice Na (mg/ℓ)||18.4||18.5||17.2||19.0||18.9||18.3||18.0||19.5||18.1|
|Wine K (mg/ℓ)||1 106||1 163||1 168||1 188||1 266||1 350||1 362||1 380||1 410|
|Wine Na (mg/ℓ)||22.9||21.9||19.9||20.9||21.8||20.6||19.8||20.7||22.1|
The juice K increase was strongly related to the amounts of K applied via the diluted winery wastewater (Fig. 1A). Juice pH increased with an increase in COD level of the diluted winery wastewater (Table 1). As a result, juice pH increased linearly with juice K (Fig. 1B). This agreed with a previous study where juice pH also increased where juice K increased due to K fertilisation (Morris et al., 1980; Morris & Cawthon 1982). It should be noted that even when 900 kg/ha K was applied to grapevines, juice pH did not exceed 3.57 (Morris et al., 1980).
Wine K tended to increase with an increase in COD level of the diluted winery wastewater (Table 1). Furthermore, wine K was linearly related to juice K (Fig. 1C). A similar trend was reported by Walker and Blackmore (2012). Wine pH tended to increase with an increase in COD level of the diluted winery wastewater (Table 1). Although wine pH was strongly affected by wine K (Fig. 1D), the pH increase did not have any negative effect on sensorially determined wine colour (Myburgh & Howell, 2014).
There were no consistent trends with regards to juice and wine Na (Table 1). However, a previous study showed that juice Na was higher where Na-based wastewater was used compared to artificial winery wastewaters with high and low K, respectively (Mosse et al., 2013). Wine Na was appreciably lower than 40 mg/ℓ to 190 mg/ℓ (Moolman et al., 1998) and 78 mg/ℓ to 533 mg/ℓ reported for Colombar and Shiraz wines, respectively (Walker et al., 2003). It is important to note that, throughout the three seasons, wine Na was considerably lower than the legal limit of 100 mg/ℓ Na for South African wines (Department of Water Affairs and Forestry, 1996).
Conclusions and recommendations
Irrigation of vineyards with winery wastewater increased juice K, which in turn increased juice pH. The same increases occurred in wine K and pH. Under the prevailing conditions, the increase in juice and wine pH did not affect wine sensorial properties. However, this does not rule out the possibility that negative juice and wine responses may be more pronounced where (i) salts accumulate more readily in heavier soils, (ii) low winter rainfall limits the leaching of salts or (iii) no cover crop is cultivated during summer to intercept salts.
Based on the project results, the following criteria should be considered for possible amendments to the General Authorisations for wineries when using diluted wastewater for irrigation of vineyards:
- The COD must be diluted to 3 000 mg/ℓ or less, preferably to less than 2 000 mg/ℓ to avoid unpleasant odours in the vineyard while irrigations are applied.
- The electrical conductivity (ECiw) must be less than 0.75 dS/m.
- The sodium adsorption ratio (SARiw) must be less than 5.
- The soil must have a low cation exchange capacity.
- The internal drainage in the root zone must be unrestricted.
- The irrigation water must not percolate beyond the root depth.
- Only micro-sprinklers should be used, since drippers have narrow flow paths and/or small orifices, and are more susceptible to clogging.
- The irrigation must be applied with micro-sprinklers in such a way that the bunches are not wetted.
- At least 50% plant available water (PAW) depletion should be allowed between irrigations to allow sufficient aeration for oxidation of organic material applied via the irrigation water.
- The irrigation frequency and volumes (schedule) should enhance, rather than negate, wine quality characteristics.
- A summer interception crop, e.g. pearl millet, should be sown in early January to ensure that its growth peaks when the winery wastewater is applied, and that the species does not complete its life cycle too early.
- Since a summer interception crop may increase the ET of vineyards substantially if growing conditions are favourable, it could induce competition for water between grapevines and cover crop. However, this can be avoided by timely slashing of the interception crop, which will also minimise possible competition for N and P.
The effect of irrigation using winery wastewater on grapevines was compared to irrigation using river water in a temperate climate with hot, dry summers. Cabernet Sauvignon/99R was irrigated using winery wastewater diluted to eight different levels of chemical oxygen demand (COD) with river water. The field trial was carried out near Rawsonville in the Breede River Valley region from 2009/10 until 2012/13. Under the prevailing conditions, juice K increased linearly with the amount of K applied via the diluted winery wastewater prior to harvest. The increase in juice K increased juice pH. Wine K increased as juice K increased. Although wine pH increased with wine K, it did not impact negatively on wine colour. It is noteworthy that wine Na did not exceed the legal limits. However, the foregoing does not rule out the possibility that negative juice and wine responses could occur (i) where salts accumulate more readily in heavier soils, (ii) where low winter rainfall limit leaching of salts or (iii) where no cover crop is cultivated during summer to intercept salts.
- The project was initiated and funded by the Water Research Commission. The project was co-funded by Winetech, THRIP TP 1208066038 and the Agricultural Research Council.
- Goudini Winery for permission to work at their winery, and in their vineyard, as well as the grapes used for samples and winemaking.
- Willie and Daniël Botha for managing the vineyard and general assistance.
- Staff of the Soil and Water Science Programme at ARC Infruitec-Nietvoorbij for technical support.
Any opinions, findings and conclusions or recommendations expressed in any publication generated through THRIP-supported research, are those of the author(s) and therefore the NRF/THRIP will not accept any liability in that regard.
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