Enhancing BOD 5 /COD ratio co-substrate tofu wastewater and cow dung during ozone pretreatment

. Ozonation pretreatment was applied to enhancing BOD/COD ratio co-substrate tofu wastewater and cow dung. Ozonation pretreatment were conducted at pH of 5.2,8.0, and 10.0, with contact times of 20, 40, and 60 minutes. The results showed that the best condition for enhancing biodegrability is pH of 10 and contact time of 60 minutes with increasing BOD/COD ratio from 0.06 to 0.49, indicating an easy biodegradable substrate category .


Introduction
The ineffective tofu industry wastewater (TW) treatment can decrease surface water quality due to its containing protein, carbohydrate, and fat respectively 40-60%, 25-50%, and 10% [1,2]. However, the low of soluble chemical oxygen demand (sCOD) concentration of the wastewater to be used in the study (3305 mg/L) makes anaerobic processing inefficient, where at least sCOD is required more than 4000 mg/L [3]. Therefore, to increase the value of sCOD, in this study the addition of cow dung (CD), which has a dissolved COD of 10,770 mg/L. However, cow dung is difficult to biodegrade because it contains cellulose, hemicellulose, and lignin respectively 25.2%, 18.6%, and 20.2% [1,4].
The biodegradability of wastewater can be known through the BOD 5 /COD ratio and wastewater characteristics test showed that the BOD 5 /COD ratio of the mixture of tofu and cow dung wastewater was 0.063 and the BOD 5 /COD ratio was <0.1 which indicated that the waste contained organic matter that was difficult to be biodegradable, could be toxic and non-biodegradable [5][6][7][8]. The minimum BOD5/COD ratio in order to wastewater easily biodegradable is 0.4 [9], and the optimal ratio is more than 0.5 [10]. Therefore, it is necessary to pretreatment the waste so that the complex structure of decomposed organic compounds becomes simpler and easily broken down by bacteria. One of the pretreatment methods is the advanced oxidation processes using ozone which will oxidize complex organic compounds into simpler compounds so that the biodegradability of the waste will increase [11].
Some researchers have proven that the pH and contact time of ozonation can increase biodegradability level of waste from 0.05 to 0.32 [12] for kraft pulp wastewater; 0.06 to 0.32 for pulp and paper wastewater [13,14]; 0.1 to 0.40 for hardwood kraft bleaching [15].
Meanwhile, based on our best literature study the effect of pH and contact time of ozonation on the BOD 5 /COD ratio of a mixture of tofu and cow dung liquid waste is unknown, so it needs to be studied. The study aimed to investigate the effect of pH of 5.2, 8.0, and 10.0 and contact time of 20, 40, and 60 minutes to determine the optimal pH and contact time for increasing BOD 5 /COD ratio from co-substrate tofu wastewater and cow dung.

Collection tofu wastewater and cow dung
The both of fresh tofu wastewater and cow dung was collected at tofu industry in Tampan Sub District of Pekanbaru City, Indonesia. Cow dung diluted using aquades at a ratio 1:2 (%, gr/gr) and the best ratio co-substrate tofu wastewater and cow dung for biogas formation of 50:50 (%, v/v) [16]. The characteristics of this substrate after mixing are shown in Table 1.

Analytical method
Ozone generation measurement by iodometry method [17] and COD and BOD analytics by SM 5520 and SM 5210 [18] respectively.  2B) removal. Both total and soluble COD removal efficiency increased gradually in the following increasing pH and contacted time, and this results in-line with previous findings of [13,14]. The longer the ozonation process, more O3 will formed and amount of OH ̊ and O 3 ions which oxidize organic substances in waste will be higher. The half-life of OH ̊ is very fast, and prolongs ozonation contact time will maintain its existence so that the degradation process of organic compounds is more optimal [19,21] or complex compounds will be more oxidized and will change in molecular weight to smaller one [21,22]. In this study, the highest percentage removal of total and dissolved COD occurred at pH 10. At alkaline pH (pH >8) the presence of OHacts as an initiator in the decomposition process of ozone to OH , in other words, the ozone solubility decreases with increasing pH [22]. When O 3 reacts with OH -, it will produce OH , O 2 , and HO 2 ̊ in the waste. OH radicals are one of the strong oxidizing agents with the oxidation potential of 2.8 V and nonselective, so any compounds that are not oxidized by ozone will be oxidized by OH radicals. OH radicals react with dissolved components and produce a series of oxidation processes until components are minimized completely.

pH vs contact time
COD parameter decreased as the effect of the presence of OH radicals. COD which consists of organic and inorganic substances will be oxidized to CO 2 and H 2 O. Oxidation process causes mineralization of organic components in the wastewater [8]. OH radical in this process was able to reduce COD with the highest values reaching 45.7% and 54.8%. At acidic pH the presence of OHis very low so that the formation of OH radicals is limited. The reaction occurs directly at acidic pH, where ozone reacts directly with organic compounds in the wastewater, but not all organic components can be completely oxidized by ozone because of its selectivity. Under acidic conditions, O 3 reacts with H + to form HO 3 , then dissociates to form OH ̊ and O 2 at a slow rate. For solutions with pH > 4 and < 10, both reactions occur directly and indirectly [24]. In this study ozonation at pH 5.22 and 8, the process occurs is directly and indirectly through the role of O 3 and OH radical, therefore

Effect pH and contact time to BOD5/COD ratio
BOD is a characteristic that shows the amount of dissolved oxygen needed by microorganisms to decompose organic compounds in aerobic conditions. Boyd (1990) emphasized that organic matter decomposed in BOD is readily decomposable organic matter, even though it states the amount of oxygen used, but it can also be interpreted that BOD is an illustration of the amount of organic matter that is easily decomposed. Fig. 3 shows the relationship between pH and contact time in the ozonation process against waste BOD.
(a) (b) Fig. 3. Effect of pH and contact time to BOD concentration (a) and BOD/COD total ratio (b). Fig. 3 shows that BOD percentage increases with increasing pH and ozonation contact time. That is because the longer time of ozone contact, more O 3 is formed and more organic compounds oxidized by ozone. The highest increase in BOD occurred in 60 minutes at pH 10 with reaching 76.02%. However, there was no significant increase in BOD at each pH. At 60 minute, pH 5.22 and 8, the increase in percentage BOD were 72.72 and 72.91% respectively. Increase in BOD caused by oxidation by O 3 and OH radicals on compounds with long molecular chains (hard to decompose) which will be converted into compounds with shorter molecular chains (easily decomposed). Oxidation process at pH 10 is occurred by use of OH radicals, so that all organic compounds present in the waste are oxidized to simple compounds, at pH 5.22 and 8, oxidation process that occurs in both, namely by using O 3 and OH radical, so the amount of organic compounds which decomposes into simple is also high. In this study, recalcitrant lignin to the biological degradation process is very easy to oxidize by ozone and OH radical. Lignin is composed of 3 types of phenylpropanoid compounds, namely coumaryl alcohol, coniferyl alcohol, and synapyl alcohol which are linked to carbon bonds (C-C) and ether bonds (COC) and have high molecular weight. All three were arranged randomly to form irregular lignin polymers.
Ozonation of lignin will change double bond in the conjugate chain to a single bond. Ozonation causes morphological changes to lignin layer so that the polymer chain broke to a hydroxyl group and induced by decreasing the bond number of the hydrogen chain [25]. According to [12], oxidation of lignin causes organic acid compounds such as muconic, maleic and oxalic acids will increase. Ozonation of protein compounds will increase the number of amino acid compounds such as cysteine, methionine, tryptophan, tyrosine, histidine, and phenylalanine [26]. Ozonation of carbohydrate compounds indirectly degrades carbohydrate compounds at a slow rate. However, decomposition products of ozone OH radicals react quickly with carbohydrates, cellulose, and hemicellulose to increase the amount of glucose in the wastewater [27]. Oxidation process causes the formation of more biodegradable compounds, so BOD becomes higher than the initial BOD of the waste. According to Mtui (2001) decrease in COD accompanied by an increase in BOD will increase the ratio of BOD 5 /COD waste. Based on Fig. 3b increase in BOD 5 /COD ratio during ozonation is directly proportional to the increase in waste pH and time of ozone contact given. BOD 5 /COD ratio before ozonation was 0.06, and increase to 0.49 during the ozonation process at pH 10 for 60 minutes. Decreasing COD and increasing BOD during the ozonization process indicate that the formation of biodegradable products [28]. Changes in biodegradability of these wastes are due to ozonation being able to change the structure of complex molecules from waste into simpler molecules [2]. Comparison effect ozonation to BOD 5 /COD ratio showed in Table 2. Table 2. Comparison ozonation process to increase BOD5/COD ratio