## 2 Pretreatment Methods for Solids Reduction
### Chemical Pretreatment
#### 2.3.1 Acidic and Alkaline Addition  
The use of acids to pretreat the wastewater sludges prior to the AD step has been studied. Acids are reported to break down recalcitrant compounds, such as lignin and cellulose, into smaller, easier to degrade by-products. Acidic environments are also favorable to organic acid degrading organisms (such as acetogens and acidogens) by allowing them to acclimate to AD conditions faster and flourish within a slightly acidic microenvironment. However, low pH levels also make the digester \"sour\" which leads to methanogenic inhibition as well as the formation of refractory compounds such as furans requiring this treatment to be paired with other pretreatment methods [14; 31]. [PERSON] et al. studied the effects pH levels within the 1-6 range using hydrochloric acid prior to the AD treatment of WAS at 37 \({}^{\circ}\)C for 21 days for batch digestion and HRT of 12 days at 35 \({}^{\circ}\)C for semi-continuous digestion [37]. Addition of an alkali after acid pretreatment is required to bring the influent sludge pH back to a neutral level prior to introduction into the AD to prevent inhibition of the microorganisms, mainly the methanogens. The results on semi-continuous digestion reported a slight to no increase in VS destruction when pretreated atpH of 2 (33% for untreated and 34% at pH 2); but they observed an increase in soluble carbohydrates and proteins by up to six-fold compared to the control. The amount of biogas produced by the semi-continuous digester digesting WAS that was pretreated at pH 2 after 36 days yielded a 14% increase in biogas methane content which suggests an increase in the bioavailability of the sludge organic fractions. Another study by [PERSON] et al. used peracetic acid (PAA) at doses ranging between 10-50 mg PAA/g SS on WAS digested at 35 \({}^{\circ}\)C for 28 days (batch) to investigate its effect on biogas production and resulting solids removal post-AD [38]. This pretreatment resulted in the biogas volume increasing by 20%, the sludge solids concentration and volatile solids concentrations decreasing by 25% and 39%, respectively, after 120 min of contact at a 30 mg PAA/g SS dose. These results suggest an enhanced biodegradation and disintegration of the WAS by the PAA treatment. The acids which are produced in-situ during anaerobic digestion can be reused which can lower the investment cost on purchasing acid for treatment. Free nitrous acid (FNA) can be produced during AD by nitration of the liquor which has proved to cause cell lysis and EPS destruction [39]. A study by [PERSON] et al. on full scale continuous WAS pretreatment by FNA for 24 h at FNA concentrations of 1.8 mg NN/l resulted in 17% of VS reduction and dewaterability increment from 12% to 14% compared to the untreated WAS. The VS reduction was supported by the increase in methane production by 16% [39]. Alkaline treatment is a well-studied chemical pretreatment method that uses alkali such as NaOH, Ca(OH)\({}_{2}\), and KOH, which aids in the breakdown of the complex pollutants and accelerates hydrolysis reactions [13; 38; 40; 41]. Solvation and saponification are the mechanisms that occur within sludge matrices via the introduction of alkalis which increases the surface area of solids making the substrate more accessible to the anaerobes [14]. The saponification of acids and esters and the neutralization of VFAs formed by the degradation of particulates during sludge transport/storage results in the increase of sCOD. A high dosage of alkali is necessary since the biomass itself consumes some amount (scavenging reactions). The high pH also aids in the lysis of the microbial cells within the sludges and increase solubility of proteins [42]. The performance of the digestion is determined by the strength of the alkali with NaOH being the strongest base and the most effective alkali for pretreatment for subsequent AD treatment from a sludge reduction perspective.