In this study, all chemical properties of the CPRS group were lower than those of the control group. In a previous study, pig slurry was reported to have a COD content of approximately 8,400 to 100,000 mg/kg and a nitrogen content of 520 to 8,800 mg/kg [
17,
18]. In this study, the COD of manure in the pit showed that manure excreted in the pig house was continuously diluted by the air-treated liquid manure. Feed rate is a critical control component when operating aerobic digestion owing to the possibility of organic loading shock [
19]. Although a high organic loading rate does not necessarily degrade the organic matter degradability, it causes the digester to become unstable. Further, aerobic granules are made well at a low organic loading rate [
20]. In CPRS, as the diluted slurry is continuously discharged, the organic loading rate decreases the inflow to the aerobic digester and thus has a positive effect on the stability of aerobic digestion. Furthermore, the application of a pit recirculation system causes a decrease in the T-N and NH
4-N content in the slurry, thereby reducing NH
3 in the air of the pig house [
21]. Although the BOD content of the anoxic tank was the lowest during the recirculation process, those of the storage tank and outlet of the slurry pit were maintained at a lower level than that of the slurry pit through dilution and degradation during the recirculation process. In this study, nitrogen was removed through nitrification and denitrification during the anoxic process [
22]. It could be explained that the T-N and NH
4-N contents of the slurry pit were greater than those of the anoxic and storage tanks. In this study, since the chemical properties of the pit manure decreased by the recirculation of air-treated liquid manure, it might be considered that the odor intensity and indoor atmosphere improved (
Table 4). Odor intensity in the CPRS group was 36.4%, 41.4%, and 51.4% lower than that of the control group in the indoor, exhaust, and outside samples, respectively. Many odorous materials of pig slurry occur in the air during aerobic digestion, which means that the amount of odorous materials decreases in aerobic-treated liquid manure. Furthermore, as aerobic treated liquid manure continuously flows to the upper layer of slurry in the pit, it has two effects: it dilutes the slurry with high organic matter content and blocks the odor generated from the surface of the slurry [
21]. In this study, as the fact that the wind velocity of the exhaust fan did not differ between the control and CPRS groups, it can be considered that CPRS has the ability to control temperature compared to the control group. Furthermore, in a previous study, it was reported that the room temperature of the Autothermal thermophilic aerobic digestion (ATAD) pit recirculation system was significantly lower than that of the control group at the same ventilation rate and under the same animal stock density conditions [
7]. In this study, the difference that can affect the environmental conditions was the animal stock density in the pig house between the treatment and control groups. However, it is considered that the temperature change due to animal stock density will be very small. Although the effect of CPRS on room temperature was compounded with the animal stock density of pig houses, a previous study [
7] and this study suggested the possibility that CPRS could control room temperature. The effect of CPRS on NH
3 reduction in pig house was reported by similarly themed papers by the same author and showed opposite results [
7,
21]. A previous study explained that the high protein feed fed to finishing pigs in the previous study led to a nitrogen flux in the recirculating liquid, finally generating NH
3 at high levels in the ATAD pit recirculation system, specifically during the summer season [
7]. This suggests that NH
3 in the pig house can be effectively reduced by controlling the amount of nitrogen supplied to the aeration tank. In this study, the equipped CPRS was a continuous type, i.e., it can continuously lower the content of organic matter flowing into the aeration tank by dilution of slurry and help to maintain low nitrogen flux during the aerobic process. Conversely, if air-treated liquid manure, which nitrogen is not sufficiently removed using the denitrification process, is inputted into the slurry pit, there is a risk of an increase in ammonia in the pig house. Therefore, the application of CPRS to piggery farms and continuous care about nitrogen is expected to reduce NH
3 emissions from the slurry in the pit. Generally, H
2S is generated under anaerobic conditions, with generation being increased under low pH conditions compared to that under high pH condition and with the main source being derived from proteins [
23]. In this study, the pH of CPRS was lower than that of the control. However, the difference in pH values was small compared with that in a previous study [
7,
21]. In contrast, the reduction effect of H
2S in the CPRS was similar to that observed in previous studies. It considers that there are two possibilities. One is considered that is the action of inhibiting anaerobic microorganisms that produce hydrogen sulfide by recirculating aerobic-treated liquid manure into a slurry pit [
24] and another one is that is considered to be a blocking effect of odor materials emission from slurry due to the air-treated liquid manure flowing at the top of the pit; most importantly, this study showed that H
2S can be reduced regardless of the pH value through the application of CPRS in pig farms. Most odorous materials are generated by microorganisms in the slurry pit. The several aims of CPRS are i) to shield odor materials coming up from the bottom by continuously flowing liquid to the top of the pit, ii) Lowering the concentration of organic matter in the pit by continuously discharging a portion of the pit manure, and iii) to increase the number of useful microorganisms instead of odor-producing microorganisms by supplying air-treated liquid manure. In this study, various odorous materials from the pig slurry pit were reduced in the CPRS group compared with those in the control and it showed the evidence that the application of CPRS in a pig house showed the reduction effect on a wide range of odorous materials. However, although CPRS is very useful for reducing odorous material from pig houses and slurry, there are still many problems to be solved. The long-term operation of CPRS can lead to problems such as the clogging of valves by salts. These problems could be solved through solid-liquid separation, advancement of the sedimentation process, and installation of strainers. In addition, the problems are caused by lack of management such as microbial imbalance, input high concentrations of nitrogen in the pit, and accumulation of the solidified slurry in the pit. These problems can worsen the odor problem in the pig house, so must be more careful. Apart from the reduction of odorous material in the pit using CPRS, there is a clear possibility that many odorous materials could still be generated in the process of manufacturing liquid manure and composting separated solids from the slurry. Nevertheless, the application of CPRS in swine farms is considered a good option as it continuously reduces the organic load of animal manure and lowers the average odorant concentration below the threshold.