Future perspectives of energy saving down-flow hanging sponge (DHS) technology for wastewater valorization—a review

Abstract

The down-flow hanging sponge (DHS) process has been rigorously studied as a significant post-treatment of anaerobically pretreated sewage and later for broader applications in industrial wastewater and pre-settled sewage treatment. The DHS has several advantages over conventional wastewater treatment technologies, i.e., simple process control, less workforce requirement, less footprint, low construction, operation, and maintenance costs due to simple design and less mechanical parts. DHS technology requires no external aeration, which saves energy use and minimizes the associated costs. Low excess sludge yield in the DHS process adds to economic efficiency. DHS achieved high organics removal efficiency up to 99% with considerable nutrient and pathogen removals at extremely short HRT. Up-to-date, six DHS configurations have been developed and extensively tested at pilot scale applications under different operational conditions, i.e., HRTs, OLRs, and SRTs. Accordingly, the DHS module has successfully been translated into full-scale demonstration plants in Japan, India, Egypt, and Thailand. This paper comprehensively reviews the mechanism and development history of the DHS system. The process performance for municipal and industrial wastewaters treatment covers a broad range of parameters, i.e., organics, nutrients, pathogens (bacteria, viruses, parasites) has been summarized. The unique feature of sludge reduction efficiency of DHS is comprehensively discussed. Moreover, the process microbiology, energy efficiency, emerging application of DHS for direct sewage treatment, pilot and full-scale experiences in the last 20 years are also reviewed. DHS technology has not yet being fully explored, mainly for nutrients removal, process modelling, optimization of wastewater and air flow configurations in DHS reactor, and biomass growth, which could help for further advancement in the process for better performance and wider applications.