The Effects Of Temperature And Light Intensity On The Performances Of Microalgal-bacterial Granular Sludge For Municipal Wastewater Treatment And Its Mechanisms

Microalgal-bacterial granular sludge（MBGS）system is a technology that uses microalgal and bacterial assimilations to treat municipal wastewater.This study investigated the performance of non-aerated MBGS process in municipal wastewater treatment at different temperatures and light intensity as well as under the natural alternate day and night conditions.The removal efficiency of organic matter,nitrogen and phosphorus in municipal sewage and the succession of microbial communities were also studied.The research results in this paper could provide theoretical basis and technical support for the further engineering application of the MBGS process.The main conclusions of this paper are as follows:（1）The 77.53±5.99%,81.93±4.35%and 86.14±2.90%of chemical oxygen demand（COD）could be removed by MBGS process at 15,22 and 30℃,respectively,indicating that a high temperature favored removal of organics due to promoted biomass growth.It was found that most of ammonia-N was removed via microbial assimilation by microalgae and bacteria in granules,with bacterial assimilation being dominant at the lower temperature.The phosphorus removal efficiency of 90.09±2.72%was achieved at 22℃,with the presence of abundant Leptolyngbyales,a potential phosphorus accumulating alga.Chlorophyta grew much faster than Leptolyngbyales at30℃ in microalgal-bacterial granules.The contributions of microalgal and bacterial assimilations toward COD,ammonia and P removal appeared to be temperature-dependent,i.e.temperature could alter the symbiotic relationship between microalgae and bacteria.（2）The MBGS at a high light intensity of 210μmol/m²/s showed optimal nutrients removal efficiencies(COD:70.50±2.84%,NH₄⁺-N:80.74±3.59%,PO₄^3--P:73.88±3.78%)compared to lower light intensities of 70 and 140μmol/m²/s.Results showed that increased light intensity could contribute to biomass growth in terms of volatile suspended solids（VSS）,chlorophyll content and extracellular polymeric substances.Illumina sequencing revealed that Rhizobium and Chlorella were the most abundant genus in bacterial and eukaryotic algal communities at the highest light intensity.Stronger light intensity was conducive to the production of oxygen by microalgae,resulting in a decrease in the abundance of anaerobic bacteria Proteiniclasticum.In addition,the metabolism and microbial assimilation of MBGS were inferred to be the main contributor to the pollutants removal,which was enhanced at high light intensity.（3）Further,outdoor experiments were explored the effect of light/temperature on the performance of MBGS.The average removal efficiency of COD、NH₄⁺-N and PO₄^3--P on day before cooling could reach 59.93%±6.79%、78.10%±7.88% and61.53%±4.54%,respectively,while the corresponding average removal efficiencies at night were 44.59%±8.01%、56.54%±17.87% and 74.21%±7.60%,respectively.The removal of NH₄⁺-N was significantly related to the average water temperature,but not related to light intensity.With the dramatic changes in environmental temperature and light intensity,the microbial biomass and stability of the system were affected to a certain extent.The removal efficiency of COD and PO₄^3--P fluctuated greatly.The reduction of chlorophyll and granule size also proved this view.Illumina Miseq showed that the decrease in the relative abundance of Clostridia and Anaerolineae of filamentous microorganisms led to smaller granule sizes.Chlorella maintained a dominant position in the eukaryotic community.