Abstract
Water quality is important to fishpond culture, and it is required to improvement in the process of aquiculture especially as it impact on normal growth for aquatic animals. Usually the purification of fishpond water adopt two methods that are in-situ and ex-situ purification, water from fishpond purified by paddy field then to return the pond belongs to ex-situ purification of fishpond water. The field experiment results showed that water form fishpond flowing through the paddy field surface more than 20%~24% of total phosphorus and 25%~33% of total nitrogen are removed which is beneficial to both fertilizer-saving for rice production and water environment improvement for pond culture, so it is liable to popularize in the middle and lower reaches of the Yangtze Rive, China. In order to extend the ex-situ purification of fishpond water by paddy field, it is needed to determine suitable size of paddy field. Based on field experiment and the results analysis, the paper gave a method to determine the length and width of the land block. The size of paddy field in long dimension is determined by flow path necessary for water quality indicators to achieve their allowable emission, the width of paddy field is calculated according to the relevant parameters including daily hydraulic load which is a total quantity of water from fishpond into paddy field, inflow rate per unit width in paddy field, a duration that fishpond water flow into paddy field for purification. Calculation and analysis showed that, for ordinary decentralized fishpond managed by peasant household, paddy field block with long side of 100 m long can satisfy the removals of TN and NH3-N contained in fishpond water through surface flow in paddy field, but can not meet the removal of TP which can be solved by flow path extension using series connection of the two adjacent plots set in standardization field block, or by setting up ecological ditch in paddy field so as to strengthen water purification.
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