## 3 Test Results and Discussion
### Water Purification Properties of the Porous Concrete Produced with (D)PAOs and Zeolite  
The water-quality measurement results of typical porous concrete and the (D)PAOs/zeolite-added porous concrete are shown in Figure 8. The initial BOD and COD concentrations of the artificial wastewater used in the present study were 9.16 mg/L and 201.04 mg/L, respectively. The results showed that the BOD and COD concentrations tended to decrease over immersion time. In the (D)PAOs-added porous concrete, the BOD concentration after seven days of immersion was found to be 3.856 mg/L, lower than that of the typical porous concrete. This phenomenon is ascribed to the existence of (D)PAOs, which decomposed and stabilized contaminants in the artificial wastewater, thereby effectively reducing the degree of contamination by organic materials. In the meantime, after seven days of immersion, the COD concentration was 20.387 mg/L, and the corresponding ratio of BOD/COD was 0.2. This ratio clearly indicated the occurrence of the biological process that eliminated organic materials capable of biodegradation [6]. Figure 8: \(Cont.\)  
Figure 7: Compressive strength of porous concretes produced with (D)PAOs and zeolite  
The T-N removal rate significantly varied between the typical porous concrete and the (D)PAOs-added porous concrete even after one day of immersion, clearly demonstrating the de-nitrification effect of (D)PAOs. The T-P removal rate was slightly higher in the (D)PAOs-added porous concrete at an early stage of immersion, but the difference significantly increased after 5-day of immersion. The reason for this is that flocs tend to increase in size and number, as shown in Figure 9, which is similarly observed when microorganisms are used to purify wastewater [11; 12; 13; 14]. Also, PAOs such as _Acetobacter_ (obligate aerobes), take in phosphorus in wastewater. Moreover, the concentrations of T-N, T-P, BOD, and COD tend to decrease because de-nitrification takes place and the BOD is consumed by microorganisms, such as _Pseudomonas_, _Acligenes_, and _Hyphomicrobium_[15; 16; 17]. The microorganisms observed using digital optical microscope (Olympus-BX51 TRF) in the test process of water purification in this study are shown in Figure 10; these microorganisms induce phosphorus accumulation and denitrification [3]. In the (D)PAOs-added porous concrete, the T-N removal rate was about 60% at 3 days old, but when both (D)PAOs and zeolite were added, the removal rate was about 65% at 3 days old, i.e., an increase of 5 percentage points. The rate slightly increased at the 5-day age, and the 7-day rate was about 86%, which was 17 percentage points higher than that of the (D)PAOs-added porous concrete. The T-P removal rate was higher in the (D)PAOs- and zeolite-added porous concrete, and there was an approximately 10% improvement over the entire course of the test. The COD removal rate was also higher in the (D)PAOs- and zeolite-added porous concrete in the early stages, and the COD concentration decreased to about 16.915 mg/L at 7 days old. The degree of BOD removal was lower than that for COD, but the BOD concentration was about 2.791 mg/L lower in the (D)PAOs- and zeolite-added porous concrete than in typical porous concrete. When compared to the (D)PAOs-added porous concrete, the BOD concentration was about 1.188 mg/L lower. Overall, the (D)PAOs- and zeolite-added porous concrete exhibited better water purification performance than did the (D)PAOs-added porous concrete because the mixing of zeolite enhances the porous concrete's ability to absorb contaminants, such as phosphorus and nitrogen, and ion exchange, accumulation, and decomposition of contaminants are promoted by the added zeolite and microorganisms [4; 5; 18; 19]. Meanwhile, to maximize the water purification effect of the microorganisms mixed into porous concrete, it is important to keep the concrete in a moist condition for a specific period of time so that the activity of the microorganisms can be enhanced.