INGENIEUR
Figure 7 : Components in POME Treatment System
are two types of discharge that are allowed by the Department of Environment ( DOE ) that may be discharged into a watercourse or for use in land applications . These have different standard discharge limits . Table 2 shows the characteristics of raw POME and standard discharge limits set by the DOE .
The POME Treatment System ( POMETS ), commonly involving primary treatment , secondary treatment and tertiary treatment is shown in Figure 7 . The primary treatment involves a physical process such as screening , decanting and oil trapping with the purpose of reducing the solid content and the substrate burden prior to secondary treatment .
The secondary treatment involves a series of biological processes which are anaerobic , aerobic and facultative , typically employing a ponding system . The anaerobic process breaks down the organic substance in the POME by anaerobic micro-organisms in the absence of oxygen and produces biogas with methane gas ( CH 4
) and carbon dioxide ( CO 2
) being the major components . The biochemical oxygen demand ( BOD ) can be significantly reduced by the anaerobic process ( Hassan et al ., 2006 ) and the BOD reduction efficiency can be up to 90 %. The anaerobic process involves four stages which are hydrolysis , acidogenesis ,
acetogenesis and methanogenesis . Each stage requires different microorganism populations to carry out the process . This process requires less maintenance cost because it is a natural process that only requires hydraulic retention time ( HRT ), a pumping system for POME transfer and circulation , which also controls other parameters such as pH , to let the micro-organisms to effectively break down the substrate . However , the aerobic process requires oxygen to degrade the organic substances in the POME . Figure 8 shows the anaerobic pond commonly used in a palm oil mill .
The tertiary treatment involves extended aeration to boost the reduction of POME ’ s BOD and also to further reduce residual suspended solid , ammonia , oil and grease as well as colour to meet the discharge standard . There are many types of tertiary treatment is used by palm oil mills such as an extended aeration system , an activated sludge system , a sequence batch reactor and a contact surface aeration system . The system can be a combination of biological and mechanical systems depending on the specification of standard requirements of the discharge set by the DOE . Commonly , the installation of a tertiary plant requires significant maintenance and running costs due to the need to install additional machinery such as an air
40 VOL 87 JULY-SEPTEMBER 2021