Effect of aeration on dissolved oxygen profile and hydration during cold soaking in paddy parboiling process

Abstract

Rice production is a key component in the Sri Lankan economy. Paddy is processed to produce raw or parboiled rice. In parboiling process, cold soaking uses about 1,300 liters of water per tonne of paddy. About 70% of the water used in parboiling with biochemical oxygen demand (BOD) ranging from 950 to 1300 mg/L is discharged as effluent into the surface waters, causing environmental damage. Anaerobic condition in the cold soaking process is considered the primary reason for high BOD of the effluent. Maintaining aerobic conditions would reduce the effluent strength. Study of dissolved oxygen (DO) profile, in the soaking water, is important in the design of an aerobic cold soaking system. An experiment was conducted to study the DO profile in conventional and aerated soaking processes using 12 month old small grain paddy. Soaking tests were 48 hour duration and DO concentration was measured intermittently. When DO reduced to about 1.0 mg/L, water was aerated by circulation through a shower, Aeration was stopped when increased to about 8 mg/L. This cycle was repeated for 48 hours. Moisture content of paddy, chemical oxygen demand (COD), and pH of effluent were measured during the experiment. Initially, it took about 9 hours of soaking for DO to deplete to lower limit and aeration to upper limit took only 25 minutes. As the experiment progressed time for DO depletion decreased and time for aeration increased. After about 21 hours, the depletion-aeration cycle stabilized at about 3 hour total cycle time. The moisture content (wet basis), COD and pH of effluent were 36.4%, 238 mg/L, and 6.83 respectively after 36 hours of soaking.