Effects Of HRTs,Flow Rates,and Sediment On Algae Growth In Reclaimed Water

In recent years,reclaimed water has become an important alternative water source for urban water.However,high nitrogen and phosphorus concentrations in recycled water easily lead to eutrophication of water body and affect it's landscape function.The growth and propagation of algae is the main reason of eutrophication.Previous studies have mainly focused on the effects of nitrogen and phosphorus on water quality,while,the hydrodynamic conditions and endogenous characteristics of the urban water have also been proved to be the important factors affecting the growth of algae,which may become the dominant factor under certain conditions.This paper aims to study the effects of hydraulic retention time?HRT?,flow velocity and sediment on algae growth urban water replenished by reclaimed water.The optimum HRT,flow velocity and sediment conditions for controlling algae growth were proposed.The correlation between nitrogen and phosphorus nutrients and algal growth index under different hydrodynamic and sediment conditions were revealed by correlation analysis.The main conclusions are as follows:?1?The growth of algae was significantly affected by HRT.When HRT was within5d,chlorophyll a increased slowly due to the dilution effect.When HRT over 10d,nutrients will be absorbed and utilized by algae,leading to the rapid accumulation of chlorophyll a.Therefore,the eutrophication of urban water can be effectively alleviated by controlling HRT,especially during the stage easier for algae growth.?2?The influence of flow velocity on algae growth is critical.In the five experimental flow rates?0.02,0.06,0.1,0.2,0.4,and 0.6 m/s?,when v?0.06 m/s,the increased flow velocity promoted the algae growth;When v?0.1m/s,the increased flow rate inhibited the growth of algae.The critical value of the flow velocity was 0.089 m/s obtained by mathematical model analysis.?3?The sediment experiment showed that the concentration of chlorophyll a in the group added sediments was 2.8 times than that in the group unadded sediments.The ammonia released from the sediments was preferentially used by algae,which promoted the rapid growth of algae.At the same time,the absorption of nitrogen and phosphorus nutrients in water by algae reproduction accelerated the further release of nutrients in the sediments.?4?Statistical analysis showed that changes in HRT,flow velocity and sediment conditions affected the correlation between total phosphorus?TP?,nitrate?NO₃^--N?and ammonia nitrogen?NH₄⁺-N?total nitrogen?TN?and chlorophyll a,which was mainly reflected in the following aspects:TP and TN have significant correlation with chlorophyll a under the condition of short HRT and high flow rate,otherwise,the correlation was not significant.The main reason is that the increase of disturbance intensity promotes the contact between algae and nutrients in the water.Reclaimed water was used to alleviate the shortage of water sources of urban water all over the world.This study focused on the effects of hydrodynamic conditions and sediments on the growth of algae,the critical HRTs and flow rates for controlling algal growth were proposed for eutrophication control and landscape function maintenance.