BACKGROUND: Metal sulfide recovery in sulfate reducing bioreactors is a challenge due to the formation of small precipitates with poor settling properties. The size of the metal sulfide precipitates with the change in operational parameters such as pH, sulfide concentration and reactor configuration has been previously studied. The effect of the hydraulic retention time (HRT) on the metal precipitate characteristics such as particle size for settling has not yet been addressed. RESULTS:The change in size of the metal (Cu, Zn, Pb and Cd) sulfide precipitates as a function of the HRT was studied in two sulfate reducing inversed fluidized bed (IFB) reactors operating at different chemical oxygen demand concentrations to produce high and low sulfide concentrations. The decrease of the HRT from 24 to 9 h in both IFB reactors affected the contact time of the precipitates formed, thus making differences in aggregation and particle growth regardless of the differences in sulfide concentration. Further HRT decrease to 4.5 h affected the sulfate reducing activity for sulfide production and hence, the supersaturation level and solid phase speciation. Metal sulfide precipitates affected the sulfate reducing activity and community in the biofilm, probably because of the stronger local supersaturation causing metal sulfides accumulation in the biofilm.CONCLUSIONS: This study shows that the HRT is an important factor determining the size and thus the settling rate of the metal sulfides formed in bioreactors.

Partial transesterification of palm oil with ethanol in diglyceride production as emulsifierHigh growth rate of palm oil production has encouraged palm diversification to be other products with higher economic value, which one of them is emulsifier. Emulsifier based on vegetable oil is biodegradable so they won’t soil our environment. Besides that, its continuity of provisioning is also ensured because it is a renewable resource. In production of emulsifier based on palm oil, transesterification reaction is a first step that influences quality of emulsifier produced. The aim of this research is to determine the condition of palm oil partial transesterification. The partial transesterification process used NaOH as catalyst and palm oil. Variables varied were percent weight of NaOH (0.1, 0.2, 0.3, and 0.4 {mole NaOH/kg oil}), transesterification temperature (40, 50, 60, and 70oC), reaction time (15, 20, 25, and 30 minutes), ratio of reactant (1:3, 1:4, 1:5, and 1:6{mole oil:mole ethanol}),  to discuss these effects to diglyceride product activity. Diglyceride product was tested by its ability as emulsifier in system stability of oil/water emulsion and to decrease surface tension of water. Result of this research indicated that diglyceride product has optimum declining of surface tension of water on percent weight of NaOH of 0.3 mole NaOH/kg oil, transesterification temperature of 50 oC, reaction time of 30 minutes, and ratio of reactant of 1:6 mole oil: mole ethanol.Keywords: diglyceride, emulsifier, NaOH, palm oil, transesterification. AbstrakLaju pertumbuhan produksi minyak kelapa sawit yang tinggi mendorong perlunya diversifikasi minyak kelapa sawit menjadi produk lain dengan nilai ekonomis tinggi, salah satunya adalah sebagai agen pengemulsi. Agen pengemulsi yang dibuat dari minyak nabati bersifat biodegradable, sehingga tidak mencemari lingkungan, dan kesinambungan pengadaannya terjamin karena berasal dari sumber daya alam yang dapat diperbaharui. Dalam produksi agen pengemulsi berbahan baku minyak kelapa sawit, reaksi transesterifikasi merupakan tahapan awal yang akan mempengaruhi kualitas produk yang dihasilkan. Penelitian ini bertujuan untuk mengkaji kondisi transesterifikasi parsial minyak kelapa sawit. Proses transesterifikasi menggunakan NaOH sebagai katalis dan minyak kelapa sawit. Variabel yang divariasikan untuk mengkaji pengaruhnya terhadap kinerja produk digliserida yang dihasilkan adalah persen berat katalis NaOH (0,1, 0,2, 0,3, dan 0,4 {mol NaOH/kg minyak}), suhu transesterifikasi (40, 50, 60, dan 70 oC), waktu transesterifikasi  (15, 20, 25, dan 30 menit), dan rasio reaktan (1:3, 1:4, 1:5, dan 1:6 {mol minyak:mol etanol}). Produk digliserida diuji kemampuannya sebagai agen pengemulsi dalam menurunkan tegangan permukaan air, serta dalam menjaga kestabilan emulsi minyak/air. Hasil penelitian menunjukkan bahwa produk digliserida memiliki kemampuan menurunkan tegangan permukaan air optimum pada persen berat katalis NaOH sebesar 0,3 mol NaOH/kg minyak, suhu transesterifikasi 50 oC, waktu reaksi 30 menit, dan rasio reaktan 1:6 mol minyak:mol etanol.Kata kunci: agen pengemulsi, digliserida, minyak kelapa sawit, NaOH, transesterifikasi.

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