The impurities from wastewater need to be removed for the equipment protection of downstream wastewater treatment facilities and efficiency as well as stability of the whole process. Especially, the increasing use of wet tissues in each household which causes clog the sewage pipe and failure of processing facilities at wastewater plants increases the operational management cost.
Sewage impurities are foreign object debris which separated with screen or general handling machine for impurities from sewage. Those are stored in arm roll box and then treated to reduce moisture content with pollinator such as sawdust as 70∼80wt% and incinerated, mostly. However, this treatment method causes deterioration of the working environment and civil complaint because high moisture content of dehydrated impurities are decayed easily and produces unpleasant smell.
Comparing with the existing incineration treatment process of sewage sludge, the cost of incineration of sewage impurities is in the range of 510,000∼540,000 won per ton which is 2.5 times higher than that of sewage sludge. Moreover, the incineration of sewage impurities produces carbon dioxide (CO2) which causes climate change in the world and inconsistent with carbon neutrality policy. Therefore, in this study, a fermentation and drying device was developed to treat sewage impurities more economical and environmentally friendly than the existing incineration process.
To testify the efficiency of fermentation and drying device to treat sewage impurities, the pilot-scale and validation-scale with a daily processing capacity of 100 kg and 300 kg, respectively, were installed and conducted experiment at a wastewater treatment plant.
It was found that the weight loss rate and water content of the fermentation dried residues from the sewage impurities ranged from 79-84 wt% and 20-25 wt%, respectively. The reduction rate of volatile solids and moisture contents were average 15% and 95%, respectively. Therefore, the fermentation and drying process was effective to remove volatile solids and moisture content in sewage impurities.
The residues could be recycled as a moisture control agent for a sewage sludge and dredged soil in a sewer and satisfied the quality standards of solid refuse fuel (SRF), meaning they could be reused as an auxiliary fuel for coal-fired electrical power plants or cement factories if SRF code for sewage impurities is enacted. The condensate water could be treated with wastewater by returning to flow control tank at a wastewater treatment facility as well.
The cost of fermentation and drying process was 90,592 won per ton which is about 78% less expensive than the existing incineration process. The odor produced during the fermentation and drying process was removed with 3 stage removal system which are zeolite, activated carbon and oxidation catalyst. The complex odor measured by air dilution function method was 144 which was satisfied the emissions acceptance criteria of 500.
It was concluded that the fermentation and drying process was much more economical and environmentally friendly than the existing incineration process mainly used to treat sewage impurities.