Effect of silica dioxide particles on the evolution of biofouling by Bacillus subtilis in plate heat exchangers relevant to a heat pump system used with treated sewage

Abstract

In 2008, a waste-heat utilization heat pump system was designed and constructed at the Beijing Olympic Village to use the energy in the treated sewage water (a bioenergy source) as a green approach for urban heat recovery. However, the treated sewage contains components that cause biofouling, such as heterotrophic bacteria, microbial nutrients and suspended substances (inorganic particles). The fouling deposited on the plate heat exchanger using the treated sewage as the heat transfer medium could sharply reduce by 60% the heat transfer coefficient in the plate heat exchanger within 40 days. Thus fouling is a composite material consisting of biological and inorganic substances. A novel combined plate heat exchanger and flow cell system was constructed to simulate the industrial treated sewage source heat pump system in the Olympic Village to better understand the biofouling characteristics. A plate heat exchanger was used to replace the actual plate heat exchangers with the same plate type, the same corrugation spacing between plates and the same material. Two flow cells with insertable studs were designed to simulate the flow in the plate heat exchanger. The effects of SiO2, the most common inorganic particles in the treated sewage, on the biofouling of the heat exchanger surface were studied. The results showed that the SiO2 particles influenced the biofilm development, further impacting the resulting heat transfer degradation. An increased biofouling mass was observed with increasing SiO2 concentration; however, the highest organic substance level in the biofouling and the highest biofouling heat transfer resistance were achieved at intermediate SiO2 particle concentrations. The inorganic particles led to expansion of the biofouling layer structure, which further affected the rate of biofouling buildup. Filaments and net biofouling structures were observed with the SiO2 particles, leading to increased heat transfer resistance.