Some authors have combined chemical addition and thermal treatment (Figure 7.1 and Table 7.1). As explained in Chapter 6, alkalis have been used to solubilize various substrates such as lignocellulosic materials and waste activated sludge (WAS) (Pavlostathis and Gossett, 1985; Ray et al., 1990). 114Alkalis allows significant solubilization and improves biodegradability performances. When combined with thermal treatment (Li and Noike, 1992; Stuckey and McCarty, 1978b; Tanaka et al., 1997), it also leads to important solubilization. However, extreme pH conditions are not compatible with anaerobic biological treatment. Penaud et al. (1999) combined NaOH addition (26.1 g/L) at 140°C for 30 min, which led to 85% chemical oxygen demand (COD) solubilization instead of 53.2% at ambient temperature. This emphasized that heating improved the pH effect. However, at high pH the biodegradability performance was limited due to the formation of refractory compounds and not due to sodium toxicity. At 4.6 g/L NaOH and 140°C, an increase of 163% in biodegradability was obtained (Penaud et al., 1999), but beyond 5 g/L the biodegradability performance decreased.115 116