Effect of pH on nutrient removal and crop production of hydroponic systems treating brewery ef ﬂ uent

The use of a crop to remove nutrients from brewery ef ﬂ uent and the in ﬂ uence of pH on these removal rates was evaluated. Cabbage ( Brassica oleracea ) was grown in recirculating hydroponic systems fed with post-anaerobically digested brewery ef ﬂ uent (BE) either subject to pH adjustment (6.5 – 7.0) or unaltered pH (8.0 – 8.5). These were compared with cabbages grown in water only and in a inorganic fertiliser nutrient solution (NS). Hydroponic systems fed with pH adjusted BE removed signi ﬁ cantly more nitrogen and phosphorus than systems fed with pH unadjusted BE ( p < 0.05). The ﬁ nal weight of cabbages from the pH adjusted BE systems were 6.7 times greater than cabbages from the pH unadjusted BE systems, whereas pH adjustment had no in ﬂ uence on cabbage weight in the water-only and NS treatments. Anaerobically digested BE that is not pH adjusted is not a suitable water and nutrient source for the hydroponic production of cabbages. However, pH adjustment of BE renders it more suitable for hydroponic crop production with hydroponic systems decreasing dissolved inorganic nitrogen, ammonium, phosphate and chemical oxygen demand concentrations by 72.8, 31.8, 98.5 and 51.0%, respectively. Hydroponic systems can be used to treat post-anaerobically digested BE to a similar standard obtained by conventional activated sludge treatment system.

Constructed wetlands are capable of providing the same treatment capabilities as AS systems (Kadlec & Wallace ). The general effect of salinity is a reduced growth rate and yield of most crops (Shannon & Grieve ). At low to medium concentrations this is primarily due to osmotic effects because of the reduced osmotic potential between the root plasma and soil water (Munns & Termaat ; Jacoby ; Mahjoor et al. ). Severity of salinity response is species specific and is also mediated by environmental factors such as humidity, temperature, wind, light and air pollution (Shannon et al. ). High temperatures and low humidity increase the effects of salinity (Shannon & Grieve ; Munns & Tester ). Salinity may cause ion toxicities and nutrition deficiencies, depending on the composition of the saline solution (Epstein & Bloom ). Chow et al. () found that the K þ requirements for shoot growth are higher under high salinities than low salinities. High concentrations of Na and Cl may accumulate in the leaves and cause 'scorching and firing' of the leaves (Shannon & Grieve ; Zorb et al. ). Calcium deficiencies are common when there is a high Na content in the soil water (Zorb et al. ). Not all salinity effects are bad, and spinach yields have been shown to increase in low to moderate salinity (Osawa ; Zorb et al. ).
Cabbage heads are more compact at low salinities but are less compact as salinity increases (Osawa ). that the optimal pH range for most plants is between five and seven. The pH of the irrigation water will therefore need to be manipulated to pH 6 to ensure that all the nutrients in BE are made available to plants.
Brewery effluent is not an ideal NS for hydroponic crop production but it has been shown to support the growth of tomatoes, lettuce and cabbage (Jones et al. ). The potential exists to use hydroponic crop production systems in replacement of conventional AS systems. This would allow the recovery of nutrients into a valuable byproduct and also decrease the energy and carbon usage of the effluent treatment system as hydroponic systems do not require large agitators and the plants are able to sequester carbon from the atmosphere during the effluent treatment process.

Aims and objectives
The aim of this study was to determine what nutrients cabbage is able to remove from BE, the influence of pH on this removal rate, and the effect of pH on crop growth and plant health in a hydroponic production system. Cabbage was grown on three different NSs, in a recirculating hydroponic system, where the pH of each NS was or was not adjusted. The objectives were to determine: (1) the effect of NS type and pH on the health, growth and leaf chemical composition of cabbage plants grown in a hydroponic production system; (2) the effect of pH on the removal of nutrients and elements from NSs by cabbages plants grown in a hydroponic production system.

Experimental species
Cabbage (Brassica oleracea cv. Star 3301) was used as the test crop because it has similar pH requirements and nutrient requirements as a wide range of vegetables (Liu et al. ). Two hundred cabbage seedlings (Starke Ayres Pty Ltd, South Africa) were purchased from a commercial seedling supplier (Moorlands Seedlings Pty Ltd, Humansdorp).
Of these, 90 similar size seedlings were used for this experiment.

Treatments
Three irrigation water sources were used that included post-PFP BE, tap water (water-only) and a conventional irrigation source consisting of an inorganic fertiliser and tap water (NS). The pH of each irrigation water source was either adjusted to 6.5 with 98% sulphuric acid (Protea Chemicals Pty Ltd, South Africa) or left unadjusted. This resulted in six irrigation treatments (Table 1). The conventional irrigation source was comprised of commercially available inorganic fertiliser (Hygrotech Pty Ltd, South Africa; Registration number K5709; Act 36 of 1947), and calcium nitrate with a composition of 11.7% nitrogen and 16.6% calcium by weight, mixed at a mass ratio of 1:0.8 and dissolved in municipal water to achieve an EC of 1,800 μm. Cabbage was grown in each treatment for 12 weeks.

Experimental system
The experiment was carried out in 18 identical recirculating hydroponic growing systems each containing five pots. Each treatment was replicated three times with a replicate consisting of an entire hydroponic system.
The system (Figures 1 and 2) was a variation of the Dutch bucket hydroponic system (Roberto ).  to the reservoir by a 20 mm plastic hose (Figure 2).
At the start of the trial, one plant was planted in each pot filled with gravel. The NS in each hydroponic system was replaced every seven days or when the water level in the reservoir was less than 25% full, whichever came first.

Data collection
Water quality parameters of different treatments were recorded before being placed in the reservoirs and prior to replacement with a new water source.

Total inorganic nitrogen (TIN) was calculated by using
Equation (1): At the beginning of the trial, the mass of each plant planted in each pot was recorded. At the end trial the mass of each plant was also recorded. The chlorophyll concentration of cabbage leaves were estimated using a handheld meter (CCM-200 Plus Chlorophyll Content

Statistical analysis
Treatment means were compared using a one-way or multifactor analysis of variance (ANOVA) and a Tukey multiple range analysis at p < 0.05. Data collected over the course of the trial were compared using a one-way or multifactor repeated measures ANOVA (p < 0.05). All data were checked for equality of variance and for the normal distribution of the residuals using Levene's test and a Shapiro-Wilk plot of the residuals, respectively. If the assumptions were not met then the data were log or square-root transformed and checked for equal variance and normal distribution of residuals. If the assumptions were still not met, a non-parametric Mann-Whitney U-test or a Kruskal-Wallis ANOVA was used to compare the data between treatments. All analyses were performed using the Statistica (version 10) software package (StatSoft Inc., Tulsa, USA).

Water quality
The average water temperature of the hydroponic systems was 21.07 C and ranged between 18.8 and 24.1 C during the experiment. At the start, post-PFP BE had the highest pH (8.36 ± 0.03) followed by tap water (7.69 ± 0.04) and then NS (7.34 ± 0.06; Table 2). The pH in all systems increased over time (Tables 2 and 3). The mean pH of the effluent systems (8.71 ± 0.06), prior to replacement, was higher than the NS (8.07 ± 0.04) or water-only treatments (8.07 ± 0.05; Table 3). Just before replacement, the pH of effluent unadjusted treatments (9.06 ± 0.06) was higher than effluent adjusted treatments (8.36 ± 0.05; Table 3).
The conductivity of BE treatments increased from 2,914 ± 47 to 3,223 ± 39 μS/cm 2 when the pH was adjusted ( Table 2). The conductivity of BE and water-only treatments did not change while the conductivity of NS treatments Values in the same row represented by a different superscript letter (a,b,c,d) represent means that are significantly different (multifactor ANOVA/Kruskal-Wallis, p < 0.05).
Treatments marked with *were subject to pH adjustment using sulphuric acid. Primary facultative pond (PFP), NS (NS), chemical oxygen demand (COD) dissolved oxygen (DO).
decreased (Tables 2 and 3). The pH adjustment of irrigation solutions had no effect on the conductivity of old irrigation solutions (Table 3).
Dissolved oxygen was not influenced by an interaction between pH regime and water source for fresh or old NSs  Table 3). The mean COD of AS-treated effluent was 16.4 ± 3.9 mg/l higher than effluent treated in hydroponic systems (Table 3). However, none of the systems was able to decrease effluent COD to discharge standards (Table 3).
The BE systems had the highest starting TIN (56.2 ± 2.6 mg/l) followed by NS (40.7 ± 1.7 mg/l) and water-only (10.1 ± 0.39 mg/l;  (Table 3). However, neither of the systems could consistently decrease the concentration of nitrate in brewery effluent to discharge standards (Table 3).
Effluent systems had the highest starting ammonium concentration (36.7 ± 2.3 mg/l) followed by NS (17.4 ± 0.85 mg/l) and then water-only (1.67 ± 0.11 mg/l; Table 2). The ammonium concentration of old irrigation solutions was similar between all experimental systems (Kruskal-Wallis, H ¼ 9.54, p ¼ 0.09; Table 3). Both the AS and hydroponic systems Values in the same row represented by a different superscript letters (a,b,c,d) represent means that are significantly different (multifactor ANOVA/Kruskal-Wallis, p < 0.05).
Treatments marked with *were subject to pH adjustment using sulphuric acid. Activated sludge ( were able to decrease effluent ammonium concentration to within the limits for discharge into a natural water resource (   Table 3). The mean phosphate concentration of AS-treated effluent was 8.29 ± 1.03 mg/l lower than effluent treated in the pH adjusted hydroponic systems (Table 3). However, neither system could consistently decrease the phosphate concentration of BE to discharge standards (Table 3).
Effluent hydroponic systems had a higher starting sodium concentration than water-only or NS systems ( Table 2). The sodium concentration increased in all systems with BE systems having the highest end sodium concentration followed by NS and water systems (Kruskal-Wallis, H ¼ 133.60, p < 0.0001; Table 3). The pH adjustment of hydroponic systems had no influence on the sodium concentration of the old NSs (Table 3). The mean starting chloride concentration of BE systems (191 ± 5.1 mg/l) was higher than NS and water only systems (104 ± 1.5 mg/l). The chloride concentration increased in all systems with BE systems having the highest end chloride concentration of 226 ± 6.5 mg/l (Tables 2 and 3). Nutrient solution and water only systems had a combined mean end chloride concentration of 128 ± 2.2 mg/l (Table 3).

Plant productivity
The final weights of cabbages were influenced by an interaction between the pH regimes and the water sources (Multifactor ANOVA, F (2,12) ¼ 85.50, p < 0.0001; Figure 3).

Corrected Proof
Cabbages from the pH adjusted BE systems were significantly bigger than cabbages from the pH unadjusted BE systems, whereas pH adjustment had no influence on cabbage size in the water only and NS treatments (Figure 3).
After 12 weeks, cabbages from the NS systems were bigger than cabbages from all other treatments (Figure 3). Similarly, the CCI of cabbages was influenced by an interaction between the pH regime and the water source (Multifactor repeated measures ANOVA, p < 0.05, Figure 4).
The CCI of cabbages grown in water only and pH unadjusted BE hydroponic systems decreased over time ( Figure 4). The CCI of cabbages grown in NS and pH adjusted BE systems increased over time, with NS grown cabbages having the highest CCI throughout the trial ( Figure 4).

Plant chemical composition
The Na leaf concentration was highest in BE grown cabbages (21.9 ± 0.41 g/kg) followed by NS (9.86 ± 0.42 g/kg) and water-only grown cabbages (3.06 ± 0.62 g/kg; Kruskal-Wallis, H (5,18) ¼ 15.74, p ¼ 0.008; Figure 5). Cabbages grown in pH adjusted BE systems had a lower Na concentration (14.8 ± 0.54 g/kg) than cabbages grown in BE unadjusted systems (29.1 ± 0.27 g/kg; Figure 5). The leaf concentration of all the measured macro-and micronutrients were significantly higher in cabbages grown in pH adjusted systems compared with plants grown in pH unadjusted BE systems, with the exception of Al, Cl and Zn (Table 4). The N concentration of cabbage leaves was similar between BE and NS grown cabbages (29.2 ± 0.5 g/kg) with water-only grown cabbages having the lowest leaf N concentration (7.9 ± 0.2 g/kg; Table 4). Nutrient solution grown cabbages had the highest P and K leaf concentration followed by BE and then water-only grown cabbages (  (Table 4).

Effluent and NS grown cabbages had higher leaf Cl and Al
concentrations than water-only grown cabbages (Table 4).

Water quality
The post-PFP BE had a high alkalinity as it took about three millilitres of 98% sulphuric acid to decrease the pH of 25 l BE from 8.41 ± 0.27 to 6.5. The pH in all pH adjusted hydroponic systems increased to a pH between 8.0 and 9.0 after seven days. The pH adjustment of BE was only done once, at the beginning of each replacement because constant pH adjustment would increase the already high conductivity of the BE systems thus putting more osmotic stress on the plants. The high alkalinity of BE is generated from the addition of sodium hydroxide to BE before it goes through high alkalinity of BE is a concern when using BE in  Values in the same row represented by a different superscript letters (a,b,c,d) represent means that are significantly different (multifactor ANOVA/Kruskal-Wallis, p < 0.05).
Treatments marked with *were subject to pH adjustment using sulphuric acid. Primary facultative pond (PFP), NS (NS).
hydroponic crop production systems because it is difficult to maintain a stable pH between 6.5 and 7.0 in hydroponic systems (Power & Jones ).
One of the main functions of the AS process is to decrease the COD effluents. The hydroponic systems were successful in decreasing the COD of BE. However, the AS and hydroponic systems were not able to decrease the COD of BE to discharge standards. The main mechanisms for COD removal in This results in a decrease in ammonium toxicity and in ammonium generated alkalinity (Gallert et al. ; Britto & Kronzucker ). Therefore, it is probably not ammonium generated alkalinity or toxicity that caused the reduction in growth of BE grown cabbages. However, the pH of hydroponic systems still rose thus indicating that other factors such as the carbon acid/base system were contributing to the high alkalinity observed in the hydroponic systems.
It is probably not the lack of nutrients in BE that resulted in the decreased plant growth but other properties such as high conductivity and pH (Jones et  There was a marked difference in the Na concentration of

CONCLUSION
The pH adjustment of post-PFP BE had a major influence on the growth, health and chemical composition of cabbage plants grown in hydroponic systems. The final weight of cabbages from the pH adjusted BE systems were 6.7 times greater than cabbages from the pH unadjusted BE systems, whereas pH adjustment had no influence on cabbage weight in the water-only and NS treatments. The macro-and micronutrient concentrations of cabbage leaves increased when the pH of post-PFP BE was adjusted to 6.5 at the start of each irrigation cycle. Post-PFP BE that is not pH adjusted is not a suitable water and nutrient source for the hydroponic production of cabbages; however, pH adjustment of BE renders it much more suitable. The pH adjustment of BE resulted in increased nutrient removal by hydroponic systems when compared to systems fed pH unadjusted BE. Hydroponic systems fed with post-PFP brewery adjusted to pH 6.5 were able to decrease dissolved inorganic nitrogen, ammonium, phosphate and COD concentrations from 54.8, 36.9, 27.3 and 216 mg/l to 14.9, 0.56, 18.6 and 105 mg/l; respectively The high alkalinity of BE is a concern, firstly for deceasing the availability of nutrients in BE, and, secondly for making it hard to maintain a pH range between 6.5 and 7.0 to optimise the availability of nutrients to the plants.
Continual pH adjustment would increase the conductivity of the BE, putting more osmotic stress onto the irrigated plants. The generation of alkalinity needs to be fully understood and technologies or practices need to be investigated that can reduce the alkalinity of BE for it to be successfully used in hydroponic crop production. Hydroponic crop production systems can be used to treat post-anaerobically digested BE to a similar standard obtained by AS treatment and future research should be aimed at identifying the most suitable crop species for this purpose.