Fluidized bed pellet reactor studies for membrane concentrate treatment

Many ethanol plants use reverse osmosis (RO) as a means of water purification. With the use of groundwater sources, calcium, magnesium and sulfate are often prevalent in high concentrations. These constituents are preserved and concentrated throughout the RO system.;Options for disposal of the concentrate stream include disposal to a receiving stream or waste system, recycling or deep well injection. Disposal to a receiving stream requires a Surface Water Discharge (SWD) Permit, and is typically controlled by the total dissolved solids (TDS) or conductivity of the water. Since a waste stream from a RO unit process contains a concentrated amount of the constituents in the feed water, it is difficult to meet typical Surface Water Discharge Permit requirements. Removal of certain constituents, in particular calcium, prior to discharge would be a beneficial practice as a method to lower TDS and conductivity.;The objectives of this research project were to optimize the chemistry of a fluidized bed pellet reactor (FBPR) for calcium carbonate removal using either calcium hydroxide (lime) or sodium hydroxide (caustic soda), to examine the effects of antiscalant usage on pellet reactor efficiency, and to develop preliminary engineering design criteria for full-scale design. The experiments for this study were conducted with a FBPR pilot plant using membrane concentrate and manganese greensand filter effluent. The performance of the pilot-scale tests were conducted at pH values of 8.5, 9.0 and 9.5 with various antiscalants.;The results of this pilot study show that conductivity can be lowered by crystallizing calcium carbonate onto a sand media. The use of caustic soda, as a pH-raising chemical, showed favorable calcium concentration reduction, with very low effluent water turbidity (<1 NTU). However, the use of caustic soda as a pH-raising chemical did not greatly lower the total conductivity of the concentrate stream because sodium was being added to the water, along with sodium carbonate in amounts equivalent to the calcium that was being removed.;The use of lime showed slightly less favorable calcium concentration reduction than caustic soda. However, the use of lime as a pH-raising chemical resulted in an elevated turbidity (>20 NTU) in the pellet reactor effluent. When lime is used instead of caustic soda, the calcium added to the water in the lime is removed prior to discharge, concurrently lowering the conductivity of the concentrate.;The presence of natural organic matter (NOM) in the source water negatively impacted the efficiency of the FBPR. Research has shown that NOM can behave similar to commercial antiscalants by sequestering calcium and prevent its precipitation as calcium carbonate. The concentration of NOM was increased through the RO process and was still present after treatment with a granular activated carbon (GAC) filter. To alleviate this problem it was discovered that the influent water needed to remain in a holding tank over night before treatment with the FBPR.