That said, review of the endless volumes of both private industry and academic whitepapers denote a very limited if not outright lack of discussion as to the wastewater that exits the digester – other then in the most rudimentary way as either fertilizer for irrigation or discharge to municipal sewerage. This lack of study and resolution does a great disservice to those considering the implementation of anaerobic digestion, as it leaves that final “10 percent” of the problem unresolved.
Of all the various published documents reviewed, only the VESP process (as a representation of reverse osmosis) has attempted to address the issue. Though RO technology has merit in addressing the problem – it is not conducive to use by the majority of farmers and small municipalities mainly due to the high power consumption – which adversely impacts the ROI (return of investment) period for the anaerobic digester. Additionally, it too must rely on either land application as a fertilizer or discharge to a municipal sewerage agency due to residual nutrients that don’t meet current Class 4 discharge to ground or surface water.
New Logic Research, Inc is the company that manufacturers the VSEP technology and it is not my intent to target them or their technology – but they do provide one of the better documents on their actual applications and the discharge characteristics of the effluent pre and post-VSEP treatment – something sorely lacking by other RO treatment providers. However, they are typical of other filtration based treatment regimes in not addressing how the concentrated reject stream is handled, nor what the options are where irrigation or municipal discharge are not viable options.
The following discussion is meant to highlight these deficiencies in system design and treatment – and maybe at least get the manufacturers to be a bit more realistic in their promotions.