As per Notification No. 11/2017-Central Tax (Rate) services falling under HSN/SAC 9988 are related to job work in specific cases and they are Manufacturing services on physical inputs (goods) owned by others. It clearly means that the job worker is not supposed to add any material on his side if he adds then it will be a supply. These services attract central tax rate of 2.5% and other services at 9%. Details are given in https://indiagstdotin.wordpress.com/2017/07/27/services-by-way-of-job-work-falling-under-hsn-sac-9988/
As per Notification No. 11/2017-Central Tax (Rate), Sewage and waste collection, treatment and disposal and other environmental protection services falling under HSN / SAC 9994 is levied at a rate of 9% for central tax.