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Effect of pH adjustment, solid-liquid separation and chitosan adsorption on pollutants ’ removal from pot ale wastewaters
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Davide Dionisi, * Materials and Chemical Engineering group, School of Engineering,
University of Aberdeen, Aberdeen, AB24 3UE, UK
Sarah Sine Bruce, Materials and Chemical Engineering group, School of Engineering,
University of Aberdeen, Aberdeen, AB24 3UE, UK
7 Malcolm John Barraclough, OMB Technology, Claylands Farm, Balfron, G63 0RR, UK
8 Abstract
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Pot ale is a wastewater from the whisky industry which is produced in large volumes and causes significant environmental concern. This study investigates the degree of COD, phosphorus, ammonia and copper removal obtained from pot ale using solid-liquid separation, carried out in the range of pH values 3.4-9.0. This study also investigates the removal of the same pollutants, from the liquid phase after solid-liquid separation, obtained by adsorption on unmodified chitosan, in a range of pH values. By solid-liquid separation, a removal of up to 14% of the COD, 60% of free phosphate, 45% of total phosphorus, 65% of ammonia and >80% of copper was obtained. In general, the highest removal of the pollutants was observed at alkaline pH values. Adsorption with chitosan, at an initial pH of the wastewater equal to 5, allowed only a modest COD removal, up to 10%, and up to 35% removal of free phosphate. When the initial pH of the wastewater was adjusted to 7, no removal of COD and phosphorus was observed with chitosan, while adsorption at more acidic pH values was impossible due to formation of a thick paste with water. Adsorption capacity for COD and phosphorus correlated well with the final pH after chitosan addition, and it was shown to decrease sharply with increasing pH. Overall, this study shows that solid-liquid separation removes a significant fraction of the pollutants in pot ale, while chitosan might only be effective after chemical modifications (e.g. cross-linking) which improve its stability at acidic pH.
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27 * email: davidedionisi@abdn.ac.uk, phone: +44 (0)1224 272814
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28 1. Introduction
29 Pot ale is a wastewater produced by the whisky industry, as the residual of the
30 distillation process (Figure 1). Pot ale is produced in millions m 3 per year and causes
31 serious environmental concerns due to the high levels of COD, BOD, phosphorus
32 and ammonia, as well as the presence of copper [1]. Current processes to dispose
33 or treat pot ale include: direct disposal to the sea, spreading on land as fertiliser,
34 evaporation to produce pot ale syrup and anaerobic digestion [2,3]. However, all
35 these methods for disposal or treatment have their limitations. Direct disposal to the
36 sea is only possible in very limited circumstances, where the location of the distillery
37 allows it. Spreading on land as fertiliser causes concern, due to the possible toxic
38 effects of the pollutants contained in pot ale. Evaporation to produce pot ale syrup is
39 expensive due to the high energy costs and the use of pot ale syrup is limited
40 because it cannot be fed to sheep due to its copper content. Anaerobic digestion has
41 large start-up costs and in general is only economically viable for large distilleries.
42 Research on pot ale treatment is mainly focussed on biological processes, anaerobic
43 digestion or sequences of anaerobic and aerobic treatment steps [4-6]. Other
44
45 alternative technologies investigated for the treatment of distillery wastewaters include coagulation-flocculation, adsorption , oxidation processes (Fenton’s oxidation,
46 ozonation and electrochemical oxidation) and membrane processes [7,8]. However,
47 none of these technologies can be considered to be totally satisfactory, and they
48 suffer from the disadvantages of the high requirement of chemicals, large sludge
49 generation and high operating costs [9].
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51
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52 This study has the following aims:
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57
58
53
54
55 a) to investigate the removal of the main pollutants in pot ale, i.e. COD, phosphorus, ammonia and copper, which can be obtained by a simple process of solid-liquid separation. The effect of pH on the degree of pollutants’ removal obtained by solid-liquid separation has been investigated; b) to investigate the removal of the pollutants by an adsorption stage carried out using chitosan.
59 In this study adsorption, rather than other chemical-physical processes such as
60 coagulation or oxidation, was considered because it is easy to implement at plant
61 scale and does not require a large capital cost. Indeed adsorption can be easily
62 carried out using self-contained units, which can be easily moved in and out of the
63 plant, and does not require the installation of any dedicated facility. This is
64 particularly important in relatively small-size plants such as distilleries. Chitosan was
65 chosen for the adsorption process because it is a cheap renewable material, which
66 has been attracting significant interest for wastewater treatment [10]. Chitosan is
67 produced from the partial deacetylation of chitin, which is produced in large
68 quantities as a waste from the seafood processing industry [11]. Therefore there is
69 increasing interest in extending the commercial uses of chitosan, since a larger
70 market for chitosan would also alleviate the waste disposal problem of the seafood
71 industry. Chitosan has potentially better adsorption properties than cellulose, which
72 is also a natural polymer potentially available from waste biomass, because of the
73 presence of amino and acetamide groups which extend the range of substances
74 which can be adsorbed on this molecule. Chitosan has been shown to remove heavy
75 metals [12], dyes [13], phosphate and nitrate [14], COD from rice mill [15] and from
76 biodiesel wastewaters [16].
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77 To the best of our knowledge, neither process investigated in this research, i.e. solid-
78 liquid separation and adsorption with chitosan, has been investigated so far for pot
79 ale wastewaters.
80
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81 2. Experimental methods
82 2.1 Pot ale wastewater and chitosan
83 A sample from pot ale wastewater was received from a distillery in Scotland and
84 used without pre-treatments.
85 Chitosan was bought from Sigma-Aldrich, product code 448877, and used without
86 pre-treatments. This chitosan is 75-85% deacetylated and has a molecular weight of
87 190,000-310,000 Da.
88 2.2. pH adjustment and solid liquid separation
89 A 2-litre sample of the pot ale wastewater was placed on a magnetic stirrer at 350
90
91 rpm. The following parameters were measured for the unmodified pot ale wastewater, as received from the di stillery (“raw” wastewater): pH, Chemical Oxygen
92 demand (COD), Total Suspended Solids (TSS), digested copper, digested
93 phosphorus, and free ammonia.
94 pH was adjusted using a NaOH solution between the original pH of the wastewater
95 (3.42) and the final value of 9.0. At the original pH and at the pH values of 5.0, 6.0,
96 7.0, 8.0 and 9.0, solid-liquid separation was carried out using filtration on Whatman
97 GF/C filter paper. Filtered samples for measurement of COD, digested copper, free
98 phosphate and digested phosphorus and free ammonia were taken from the filtrate
99 at each pH value.
100 2.3 Adsorption experiments
101 Adsorption experiments were carried out using pot ale after pH adjustment and
102 filtration. The pH of the initial wastewater was adjusted to values of 5.0 and 7.0. The
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103 adsorption experiments were conducted in 500 ml glass bottles with 50 ml of filtered
104 sample in each, to which various concentration of chitosan were added. A control
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106 experiment was also run with no chitosan added. The bottles were agitated in an orbital shaker running at 200 rpm at a temperature of 20 °C for 18-20 hours to
107 ensure that equilibrium had been achieved. After adsorption had taken place, each
108 bottle was re-filtered using the Whatman GF/C filter paper to ensure all chitosan was
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110 removed. The following parameters were measured and compared with the results from the “control” bottle: COD, free phosphate and digested phosphorus, free
111 ammonia and digested copper. An additional experiment was carried out where the
112 pH of the wastewater was controlled to the value of 6, by adding the appropriate
113 concentration of sulphuric acid after chitosan addition before proceeding with the
114 adsorption experiment. The adsorption capacity q e

 mg adsorbate g chitosan

 was calculated as
115 follows:
116 q e

C
0
 m
C
117 where C
0
is the initial concentration of the adsorbate (COD or free phosphate), C is
118 its final concentration at the end of the adsorption experiment and m is the
119 concentration of chitosan in the experiment.
120 2.4 Analytical methods
121 COD, phosphorus, ammonia and copper were measured using the appropriate
122 Spectroquant Cell test method (Merck Millipore, method number 114555 for COD,
123 100673 for phosphorus, 114558 for ammonia and 114553 for copper), and the
124 Spectroquant Nova 60 photometer. For the determination of digested phosphorus,
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125 samples were digested for 30 min at 120 O C, according to the procedure described
126 in the Spectroquant cell test method. For copper analysis, samples were pretreated
127 as follows: 0.1 ml HNO
3
/ml sample was added, then the samples were digested at
128 100 O C for 1 h. After cooling to room temperature, the copper concentration was
129 measured.
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131 3. Results and discussion
132 The initial composition of the pot ale sample used in this study is reported in Table 1.
133 3.1 pH adjustment and solid-liquid separation
134 3.1.1 Base addition and pH increase
135 Figure 2 shows the increase in pH in pot ale sodium hydroxide. When the pH
136 reaches a value of approximately 5, pH starts to increase more rapidly. The slow
137 increase of pH at the beginning is probably due to the presence of weak acids in pot
138 ale, which have a buffering effect on pH. The presence in pot ale of volatile acids,
139 including, among the others, acetic, propionic and lactic acid, has been reported [1],
140 at a total concentration up to 10 g/l. The maximum pH in our tests was set at the
141 value of 9 because this is the maximum pH limit for the discharge of pot ale effluents
142 for the distillery which provided the pot ale sample.
143 3.1.2 COD removal
144 Figure 3 shows the COD profile both in the raw pot ale sample at the initial pH and in
145 filtered samples as a function of pH. It can be observed that the raw value is greater,
146 by approximately 10%, than the filtered value for the same pH, indicating that some
147 proportion of COD is insoluble and bound to the solids. This means that by simply
148 removing the solids from the raw pot ale wastewater a 10% decrease in COD may
149 be achieved. As the pH value is increased, a slight decrease, corresponding to about
150 4% from the lowest to the highest pH value, in the filtered COD levels is observed.
151 Overall, by removing the solids and increasing the pH a reduction of COD of
152 approximately 14% can be achieved. Our results are in qualitative agreement with
153 the results by Tokuda et al . [17], which observed a 20% removal of COD from
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154 untreated pot ale, at the original acidic pH, by sedimentation. COD removal due to
155 solid-liquid separation can be explained by the organic nature of the solids in pot ale,
156 which contain dead yeast cells, proteins, etc [17,18].
157 3.1.3 Phosphorus removal
158 Figure 4 shows digested phosphorus and free phosphate profiles in the raw pot ale
159 and in filtered samples in a range of pH values. At the initial pH, total phosphorus in
160 the raw pot ale and in filtered samples was virtually the same, therefore indicating
161 that there is virtually no phosphorus bound to solids in the raw pot ale. This is in
162 agreement with what observed in another study [17], where only 5% decrease in
163 total phosphorus was observed by removing the solids using sedimentation.
164 The difference between digested phosphorus and free phosphates is due to soluble
165 phosphorus-containing species different than phosphates, e.g. polyphosphates. Both
166 free phosphates and digested phosphorus in the filtered samples showed a marked
167 decrease at pH 9. This is likely due to phosphorus precipitation. Interestingly, the
168 difference between digested phosphorus and free phosphates does not change
169 significantly at pH 9 compared to the other pH values, and this means that probably
170 only free phosphates precipitate, while other forms of soluble phosphorus remain in
171 solution. Overall, by adjusting the pH and separating the solids, a reduction in free
172 phosphate of approximately 60% and in total phosphorus of approximately 45% can
173 be achieved. The most likely forms for phosphorus precipitation in this study are as
174 calcium or magnesium salts. Calcium is likely to be present in pot ale, due its
175 presence in the starting material, barley, in the yeasts and in the water used for the
176 whisky production process. Satyawali and Balakrishnan [9] report a calcium
177 concentration of 0.8 and 0.2% in cane and beet molasses, respectively. Even though
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178 these data are not immediately transferable to pot ale, they give an indication of the
179 likely presence of calcium in pot ale at significant concentrations. Phosphorus may
180 precipitate as different species of calcium phosphates, of which hydroxyapatite
181 Ca
5
(PO4)
3
OH is thermodynamically the most stable one [19]. Precipitation as
182 calcium phosphate is favoured by alkaline pH values [20] and this may explain the
183 sharp drop in phosphorus concentration observed at pH 9. The reaction of
184 phosphorus precipitation as hydroxyapatite can be described by the stoichiometry
185 below [20]
186 5 Ca
2
 
3 PO
4
3
 
OH
 
Ca
5
( PO
4
)
3
OH (1)
187
188
Another possibility is phosphorus precipitation as magnesium salt, i.e. as struvite,
NH
4
MgPO
4
·6H
2
O. Mg has been reported to be present in pot ale at a concentration
189 of approx. 0.2% [17]. The effect of pH on struvite precipitation is complex and is
190 dependent on the presence of other ions, however in general struvite precipitation is
191 favoured by alkaline pH values and is described by the stoichiometry below [21]
192 Mg
2
 
NH
4
 
PO
4
3
 
6 H
2
O

MgNH
4
PO
4

6 H
2
O (2)
193 Struvite formation from pot ale after anaerobic digestion was observed by Tokuda et
194 al . [17], who observed more than 90% phosphate removal by adding a magnesium
195 salt in a process carried out at pH 8.2-8.4.
196 3.1.4 Ammonia removal
197 Figure 5 shows the profile of free ammonia as a function of pH. Ammonia
198 concentration remains virtually unaffected by pH until pH 8, and then it shows a
199 sharp drop at pH 9, with approx. 65% ammonia removal. The profile of ammonia as
200 a function of pH can be explained, similarly as what discussed in a previous section
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201 for phosphorus removal, by ammonia precipitation as struvite, described by equation
202 (2), which is favoured by alkaline pH values. Ammonia removal as struvite precipitate
203 has been reported in many instances in the literature [22]. Using anaerobically-
204 treated swine waste, Miles and Ellis [23] reported a maximum of approximately 90%
205 ammonia removal as struvite by adding external magnesium and phosphate. The
206 optimum pH was found to be 9.5. Since struvite formation is often reported to be
207 limited by the amount of magnesium ions available, an additional experiment was
208 carried out by adding an external source of magnesium to the pot ale at pH 9, in
209 order to determine whether this would cause further precipitation of ammonia.
210 However, the results showed that the free ammonia concentration did not decrease
211 further, therefore indicating that ammonia precipitation is not limited by the
212 availability of magnesium ions.
213 3.1.5 Copper removal
214 Figure 6 shows the profile of digested copper as a function of pH. In general the
215 digested copper profile is somewhat scattered, but generally, the concentration of
216 digested copper decreases with an increase in the pH. The issue with scattered data
217 could be linked to analytical issues arising due to the complex organic matrix of the
218 pot ale wastewater and the strong colour of the sample which would have had a
219 detrimental effect on the photometer reading. At the original pH a considerable
220 difference in the raw and filtered sample readings can be observed. From filtering
221 alone, a reduction of almost 50% of copper is achieved which suggests that a high
222 proportion of copper in the pot ale wastewater is insoluble and bound to solids. This
223 is agreement with other studies reported in the literature. Graham et al . [1] found
224 total concentration of copper in pot ale to be in the range 2-5 mg/l, of which on
225 average less than 50% was in soluble form. Quinn et al . [24] found total copper
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226 concentration in pot ale in the range 2.1-2.3 mg/l, of which a fraction varying in the
227 range 20-70% was in soluble form. By increasing the pH to 8 or above, only a very
228 low copper concentration was left in the filtrate, so indicating that, by a process of
229 solid-liquid separation coupled with pH adjustment, more than 80% of the copper in
230 pot ale can be precipitated out of the solution. Our results are consistent with the
231 hypothesis that copper is removed via precipitation as hydroxide. The reaction of
232 copper hydroxide precipitation at alkaline pH can be described by the reaction below
233 Cu
2

( aq )

2 OH

( aq )

Cu
 
2 ( s )
(3)
234 The reaction shows that, increasing the pH, the concentration of copper in solution
235 decreases, in agreement with our findings. However, it should be noticed that the
236 chemistry of copper in water is very complex and many other reactions can also
237 occur, especially in complex matrices such as pot ale.
238
239 3.2. Adsorption on chitosan
240 3.2.1 Effect of chitosan on pot ale pH
241 In the adsorption experiments it was observed that the pot ale pH increased due to
242 the addition of chitosan. Table 2 shows the final pH after adsorption tests with the
243 various concentration of chitosan tested, for two values of the initial pH of the pot ale.
244 The increase in pH was dependent on the chitosan concentration and was
245 particularly important for the tests with initial pH of the pot ale equal to 5. In this case,
246 pH increased by almost 2 pH units (up to 6.88) at the highest chitosan concentration
247 tested. The increase in pH was due to the protonation of the amine group on the
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248 chitosan molecule, which caused a removal of hydrogen ions, according to the
249 following reaction [12] :
250 RNH
2

H
 
RNH
3

(4)
251 where RNH
2
is the amine group on the chitosan molecule.
252 The increase in pH with chitosan addition had important effects on the adsorption
253 results, as discussed in the next sections, which report the results for the
254 experiments with initial wastewater pH equal to 5.
255 3.2.2 COD removal (pot ale at initial pH 5)
256 Figure 7 shows the residual COD in the pot ale as a function of the chitosan
257 concentration. Organic species, measured as COD, can be removed by chitosan by
258 two main mechanisms: electrostatic adsorption on the protonated amino group
259 and/or binding to the hydroxyl group [25-27]. At a chitosan concentration of 10 g/l
260 approximately 10% of the initial pot ale COD was removed. However, very little
261 further improvement is observed for chitosan concentrations higher than 10 g/l. For
262 the highest chitosan concentration tested, 50 g/l, a lower COD removal is observed
263 than at 10 and 20 g/l. The initial part of the curve of the residual COD as a function of
264 the chitosan concentration can be simply explained by the higher COD removal that
265 is obtained by increasing the adsorbent concentration, as expected in adsorption
266 processes. However, the fact that the residual COD did not decrease further at
267 higher chitosan concentrations was not expected according to the standard
268 adsorption theory and was probably due to the higher pH in the experiments at
269 higher chitosan dosage. As shown by equation (4) in section 3.2.1 and as further
270 discussed in section 3.2.6, a higher pH causes a lower degree of protonation for the
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271 amino groups on the chitosan molecule and this may cause a lower adsorption. The
272 worse performance of higher chitosan concentrations on COD removal is confirmed
273 by the adsorption isotherm, showing the adsorption capacity vs. the equilibrium COD
274 concentration. The unusual profile of the adsorption isotherm is due to the lower
275 COD adsorption observed at the highest chitosan concentration tested, which was
276 linked to the higher pH. The unusual profile of the adsorption isotherm prevents a
277 simple mathematical description of adsorption, e.g. a Langmuir or Freundlich
278 isotherm. On the other hand, the maximum adsorption capacity, observed with the
279 lowest chitosan concentration tested (1 g/L) was approx. 900 mg COD/g chitosan, in
280 the same range of the adsorption capacity of chitosan for COD for other wastewaters
281 reported in the literature. For example, Pitakpoolsil and Hansom [16] observed an
282 adsorption capacity in the range 1000-6000 mg COD/g chitosan from a biodiesel
283 wastewater using unmodified chitosan flakes. In a study on rice mill wastewater [15],
284 the adsorption capacity of chitosan for COD was found to be in the range 1000-4000
285 mg COD/g chitosan [14,15]. Using diluted vinasse [28], chitosan adsorption capacity
286 for COD was found to be in the range 200-500 mg COD/g chitosan. This comparison
287 with literature studies shows that the maximum adsorption capacity of chitosan for
288 COD in pot ale is comparable to the chitosan adsorption capacity for other effluents,
289 therefore indicating that chitosan is potentially able to remove COD from pot ale.
290 However, the high initial COD in pot ale and the increase in pH observed with high
291 chitosan dosages with consequent decrease in adsorption capacity, allowed for only
292 a modest COD removal, up to 10%, in this study.
293 3.2.3 Phosphorus removal (pot ale at initial pH 5)
294 Figure 8 shows total phosphorus and free phosphate after adsorption with chitosan
295 at different concentrations. Phosphate removal by chitosan is usually explained on
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296 the basis of the electrostatic interactions between the negative phosphate ions and
297 the protonated amine groups in the chitosan molecule [14,29]. To this regard, it is
298
299 important to observe that in the whole pH range investigated in this study phosphate is present as negative ions, either H
2
PO
4

or HPO
2
4

. The trend of phosphate as a
300 function of chitosan concentration is similar to what observed for COD, with a sharp
301 decrease in adsorption capacity at chitosan concentrations larger than 10 g/l, and
302 this can again be linked to the increase in pH. The adsorption isotherm confirms the
303 decrease in adsorption effectiveness at higher chitosan dosages. The maximum
304 adsorption capacity for free phosphates and for soluble phosphorus observed in this
305 study is approximately 30 mg P/ g chitosan. The adsorption capacity of chitosan for
306 phosphate measured in this study is in good agreement with the adsorption capacity
307 for free phosphate, on synthetic solutions, of quaternized chitosan beads, which was
308 found to be in the range 10-60 mg P/ g chitosan [14]. The conclusion from the results
309 of phosphate adsorption are similar to what we observed for COD in the previous
310 section: chitosan can be effective in removing phosphate from pot ale, however the
311 maximum degree of phosphate removal is limited by the pH increase caused by high
312 chitosan dosages, which decreases the adsorption capacity for phosphate. This is
313 further discussed in section 3.2.6.
314 3.2.4 Ammonia and copper removal (pot ale at initial pH 5)
315 Ammonia was not removed at any chitosan concentration. This was probably due to
316 the competition of other species for the active sites on chitosan. Indeed, literature
317 studies have shown that ammonia can be removed by chitosan from synthetic
318 solutions. Using crosslinked chitosan, an adsorption capacity of up to 120 mgN/g
319 solid was observed [30] for synthetic solutions of ammonia in the wide pH range 4-9.
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320 Copper was virtually completely removed at all the chitosan concentration tested, as
321 expected on the basis of the low copper concentration in the wastewater. The
322 observed copper removal by chitosan is in agreement with various literature studies
323 which report the ability of chitosan to remove copper. An adsorption capacity in the
324 range 20-80 mg Cu/g chitosan was observed for copper solutions at pH 6[31]. In
325 another study, the maximum adsorption capacity of crosslinked chitosan for copper
326 was 318 mg Cu/g solid [32]. The mechanism for copper removal by chitosan is
327 generally accepted to be binding to the amino group [33].
328 3.2.5 Colour removal (pot ale at initial pH 5)
329 Figure 9 shows the effect of chitosan concentration on absorbance at four
330 wavelengths. In general, at all the wavelengths absorbance decreases with
331 increasing chitosan concentration, indicating that colour is removed by chitosan.
332 Colour removal by chitosan can be explained by adsorption of organic species
333 containing chromophore groups and/or by the removal of transition metals, which
334 can be responsible for colour in wastewaters. Colour removal by chitosan has been
335 shown in several literature studies [11,28,34,35].The ability of chitosan to decrease
336 colour from pot ale is particularly important, since usually biological processes are
337 not able to remove colour from distillery wastewaters [9]. Very few studies have
338 investigated colour removal in pot ale, e.g. Tokuda et al .[17] investigated colour
339 removal using coagulation with FeCl
3
followed by sedimentation.
340 3.2.6 Experiments at different pH
341 The results reported in the previous section on adsorption at initial wastewater pH
342 equal to 5 indicate only a limited degree of COD and phosphorus removal.
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343 Therefore, additional experiments were carried out to investigate whether a change
344 in pH could determine an improvement in the adsorption capacity.
345 An experiment was carried out with the wastewater at initial pH 7 (the final pH was
346 higher than 7, due to the chitosan addition, as described in section 3.2.1). No
347 adsorption of COD, phosphorus and ammonia was observed. The results indicate
348 that increasing pH has a negative effect on adsorption with chitosan and this is
349 probably due to the fact that at higher pH the NH
2
group on the chitosan molecule is
350 less protonated, and therefore less effective in adsorption, than at lower pH. Indeed,
351 chitosan adsorption is usually reported to be less effective at higher pH values due to
352 the lower protonation of the amine group and consequent decrease in electrostatic
353 interactions between chitosan and organic molecules [11]. In a study from biodiesel
354 wastewater, a decrease in adsorption capacity of chitosan for COD was observed
355 when pH increased above 4 [16]. Adsorption capacity for dyes of cross-linked
356 chitosan was shown to decrease markedly with increasing pH [36] and, similarly,
357 adsorption of humic acids on chitosan was shown to be more favourable at pH 6.5
358 than 8.5 or 12 [37]. For free phosphate, the adsorption capacity of cross-linked
359 chitosan beads was observed to decrease with pH for pH higher than 6 [14].
360 Figure 10 shows a more quantitative analysis of the effect of pH on COD and free
361 phosphate adsorption. Figure 10A, obtained combining the results of the
362 experiments at initial pH 5 and 7, shows the adsorption capacity for COD and free
363 phosphate as a function of the final pH of the slurry after chitosan addition. A sharp
364 decrease in adsorption capacity with increasing pH was observed. Figure 10B shows
365 the theoretical ratio between protonated amine groups and total amine groups on
366 chitosan as a function of pH. The curve was calculated assuming the equilibrium
367 reaction (4), section 3.2.1, with an equilibrium constant equal to 10 6.2 [12]. The curve
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368 shows that at pH 5 virtually all the amine groups are protonated, but the degree of
369 protonation decreases sharply with increasing pH and at pH higher than 7 the
370 degree of protonation is very low. Figure 10C combines the information from Figures
371 10A and 10B and shows the adsorption capacity as a function of the fraction of
372 protonated amine groups. It is evident that for both COD and phosphate the
373 adsorption capacity is well correlated with the fraction of protonated amine groups,
374 showing a strong increase as the fraction of protonated groups increases. Overall,
375 this analysis indicates the strong effect of pH on adsorption of COD and phosphate
376 from pot ale and highlights the benefit of working at acidic pH.
377 In order to determine whether better adsorption performance could be obtained at
378 lower pH, an experiment was carried out whereby the pH was controlled at the value
379 of 6.0 by sulphuric acid addition. However, under these conditions at most of the
380 chitosan dosages tested (10, 20 and 50 g/l) swelling of chitosan was observed and
381 chitosan formed a thick paste with water, therefore preventing any solid-liquid
382 separation.
383 The swelling of chitosan observed in the experiment at controlled pH 6.0 is in
384 agreement with what observed in the literature. Indeed, it is generally observed that
385 unmodified chitosan loses its integrity at acidic pH due to the protonation of the
386 amine group, which causes partial dissolution [11]. This effect has been observed in
387 various experimental studies [12,13].
388 In summary the results of our study indicate that unmodified chitosan gives a better
389 adsorption of COD and phosphorus in pot ale when the final pH of the slurry (after
390 chitosan addition) is in a narrow pH range, close to 6.3. At higher pH values the
391 adsorption capacity decreases, likely due to the lower protonation of the amine
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392 group, while at more acidic pH values chitosan swelling and dissolution prevents any
393 solid-liquid separation, especially at high chitosan concentrations. Therefore,
394 unmodified chitosan cannot be efficiently used for COD and phosphorus removal
395 from pot ale. The use of cross-linked chitosan, which has been shown to have better
396 mechanical properties and better stability to acidic conditions [31], needs to be
397 investigated.
398
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399 4. Conclusions
400 This study shows that by adjusting pH and separating the solid and liquid fractions of
401 pot ale the following removal of the main pollutants can be achieved: COD 14%, free
402 phosphate 60%, total phosphorus 40%, ammonia 65%, copper >80%. Phosphorus
403 and ammonia are only removed at pH 9, the highest investigated pH value, and this
404 indicates that they are probably removed by precipitation. Copper is removed when
405 pH is higher than 7, and this is also likely due to precipitation. The majority of the
406 removed COD can be removed even at the original acidic pH of the pot ale, so
407 indicating that a fraction of the total pot ale COD is bound to the solids.
408 Chitosan shows a good adsorption capacity for COD and phosphorus in pot ale, in
409 line with other experimental studies on different wastewaters or synthetic solutions.
410 However, the overall removal of COD and phosphorus that can be obtained with
411 unmodified chitosan is limited to approximately 10% of COD and 35% of
412 phosphorus. The reason for this is the decrease in adsorption capacity which is
413 observed when pH is higher than approximately 6.3 and the instability of chitosan at
414 pH values of 6.0 or lower. The use of chemically modified or cross-linked chitosan,
415 with better mechanical properties and better resistance to acidic pH values, might
416 offer a more effective treatment method for pot ale and deserves further
417 investigation.
418
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419 Acknowledgment
420 The authors acknowledge the financial support of the Energy Technology
421 Partnership (ETP) under the ETP Consultancy Fund. The authors also acknowledge
422 the skilful assistance of Mrs Liz Hendrie in performing the experiments.
423
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424 References
425
426
427
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[2] F. Jack, J. Bostock, D. Tito, B. Harrison, J. Brosnan, Electrocoagulation for the removal of copper from distillery waste streams, Journal of the Institute of Brewing
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431
432
[3] G. Newbert, Energy efficient drying, evaporation and similar processes, Journal of
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526
Table 1. Composition of the pot ale used in this study
Parameter pH
TSS
Raw COD
Filtered COD
Filtered ammonia
Raw digested phosphorus
Filtered digested phosphorus
Filtered free phosphate
Raw digested copper
Filtered digested copper
Value
3.4
5.3
83.4
75.9
0.3
0.7
0.7
0.5
1.3
0.3
Units g/l g/l g/l gN/l gP/l gP/l gP/l mg/l mg/l
26

527
528
529
530
Table 2.
Effect of chitosan concentration on the final pH, for two values of the initial pot ale pH.
Chitosan dose
(g/l)
0
1
10
20
50
Final pH for the wastewater at initial pH 5.03
4.98
5.26
6.27
6.50
6.88
Final pH for the wastewater at initial pH 7.06
7.12
7.29
7.24
7.33
7.46
27

545
546
547
548
549
550
539
540
541
542
543
544
531
532
533
534
535
536
537
538
551
552
553
Water
Yeast
Barley
Malting
Malt
Mashing
Wort
Fermentation
Wash
Distillation
Low wines
Second distillation
(Spirit still)
Spirit
Maturation
Whisky
Draff
Pot ale
Spent lees
Figure 1 . Main stages in the whisky production process. Pot ale, target of this study, is circled.
28

554
555
556
10.00
9.00
8.00
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
0 0.02
0.04
0.06
0.08
0.1
Figure 2.
Base concentration required to increase pH of pot ale.
29

90000
80000
70000
60000
50000
40000
30000
20000
Filtered
Raw
10000
0
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
pH
557
558
559
Figure 3.
Raw pot ale COD and filtered values as a function of pH.
30

800
700
600
500
400
300
200
100
Free phosphate-filtered
Digested Phosphorus-filtered
Digested phosphorus-raw
560
561
562
563
0
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
Figure 4. Raw phosphorus, filtered digested phosphorus and free phosphate as a function of pH.
31

564
350
300
250
200
150
100
50
565
566
567
0
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
Figure 5.
Free ammonia as a function of pH.
32

568
3.00
2.50
2.00
1.50
1.00
Filtered Raw
0.50
569
570
571
0.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
Figure 6.
Copper in raw and filtered pot ale as a function of pH.
33

572
573
574
575
576
577
77000
76000
75000
74000
73000
72000
71000
70000
69000
68000
67000
0
A
10 20 30 40
50 60
1000
900
800
700
600
500
B
400
300
200
100
0
66000 68000 70000 72000 74000 76000
Figure 7.
A) Residual COD after adsorption with chitosan at various concentrations of chitosan. B) adsorption isotherm. Experiments with initial wastewater at pH 5.
34

800
700
600
500
400
300
200
100
0
0 free phosphate digested phosphorus
A
10 20 30 40
50 60
578
35
30
25
20
15
B
10
5
0
300 350 400 450 500 550
e
579
580
581
582
583
Figure 8.
A) Residual phosphorus and phosphate after adsorption with chitosan at various concentrations of chitosan. B) adsorption isotherm. Experiments with initial wastewater at pH 5.
35

584
1.6
1.4
1.2
1
0.8
400 nm
600 nm
500 nm
700 nm
0.6
0.4
0.2
585
586
587
588
0
0 2 4 6 8
10 12
Figure 9.
Colour removal after adsorption with chitosan. Experiments with initial wastewater at pH 5.
36

1000
900
800
700
A
35
30
600
500
400
COD free phosphate
25
20
15
300
200
100
10
5
0
5.00
5.50
6.00
6.50
7.00
7.50
8.00
final pH
0
589
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
5
B
590
5.5
6 6.5
pH
7 7.5
8
1000
900
800
700
600
500
400
300
200
100
0
0.00
COD
Free phosphate
C
5
1.00
0
15
10
35
30
25
20
591
0.20
RNH
3
+
0.40
0.60
/(total RNH
3
+ +RNH
2
)
0.80
592
593
594
595
Figure 10.
Effect of pH on adsorption. A) Adsorption capacity as a function of final pH for COD and phosphorus; B) Theoretical fraction of protonated amine groups on chitosan as a function of pH; C) Adsorption capacity for COD and phosphorus as a function of the fraction of protonated amine groups on chitosan.
37