Optimization of Ozonation Process for the Reduction of Excess Sludge Production from Activated Sludge Process of Sago Industry Wastewater Using Central Composite Design

Table 2

Level of factors and their values used for the experiment.

Variables Coded values	Actual values for the coded values
	−1.682	−1	0	+1	+1.682
pH (𝑋1)	4	5.2	7	8.8	10
Ozonation time (𝑋2) (minutes)	1	4.9	10.5	16.1	20
Retention time (𝑋3) (days)	1	2.8	5.5	8.2	10