© 2019 Author(s). The purpose of this research is to design gravity thickener as appropriate equipment in conversion process of palm oil mill effluent (POME) to biogas at pilot scale in order to obtain high decomposition rate of VS. POME was taken from a fat pit of waste treatment unit belong to Adolina Mill PTP Nusantara IV. The experiment was conducted in a pilot-scale biogas power plant located at University of Sumatera Utara where recycling sludge was carried out using a gravity thickener made of carbon steel equipped with 5 faucets as sampling point. Various types of chemicals were used in this study such as NaHCO 3 , FeCl 2 , NiCl.6H 2 O and CoCl 2 .6H 2 O. pH digester was maintained in the range of 6.5-7.8 while M-alkalinity was controlled ≥ 3.000mg/l with the addition of NaHCO 3 of 2.5g/L POME. The experiment was started with loading up until it reached the target HRT of 6 days. Measurements of TS and VS were performed on various gravity thickener diameters, recycle sludge percentages and sludge levels until they achieved stable data. Experiments with varying gravity thickener diameter of 315 and 350mm showed no change in TS and VS concentration significantly. Furthermore, biogas production increased with the increasing of recycle sludge. Samples were taken under sludge level variations at 130, 150, 170, 190 and 210cm from the bottom of gravity thickener. In this study, experiments were performed with and without annulus inside the gravity thickener. However, the determination of sludge level could not be carried out, since TS and VS levels at each sludge level which could not be constant due to the turbulence. TS and VS at sludge level 130 and 150cm inside the gravity thickener equipped with annulus were still very high, ranging from 23,000 to 26,000mg/l. A series of experiments showed that gravity thickener diameter of 315mm, recycle sludge 50% equipped with annulus could produce VS decomposition rate up to 77%.