生物质灰致沼液氮磷脱除研究

Ammonia Nitrogen and Phosphorous Removal from Biogas Slurry Induced by Biomass Ash Addition

为降低沼液氮磷脱除的操作费用,提出向沼液中添加生物质灰来辅助氮磷脱除。试验研究了水稻秸秆、花生壳、棉花秸秆和玉米秸秆4种生物质在600℃下燃烧后的生物质灰对沼液pH值、氨氮浓度和总磷的影响。同时考察了生物质灰添加对沼液化学需氧量、悬浮物质量浓度、浊度及植物生理毒性的影响。结果表明,棉花秸秆灰在沼液中的溶解度最大,为12.97%,而花生壳灰溶解度最低,仅为10.67%。通过Ca^(2+)、Mg^(2+)和OH^-等离子的引入,生物质灰添加可部分沉淀沼液中的CO_2,提升沼液p H值至9.5~11.0,满足"热-吹脱"氨氮脱除工艺对p H值的要求。同时,随着生物质灰添加量的增加,沼液中总磷含量基本呈现先降低后略微升高的趋势,当添加100 g/L棉花秸秆灰时,沼液总磷最高脱除率可达78.74%,其质量浓度可由初始值19.66 mg/L降低至4.18 mg/L。这表明生物质灰添加有利于氮磷脱除,理论上可降低氮磷脱除的操作费用。另外,添加生物质灰可在一定程度上降低沼液化学需氧量、悬浮物质量浓度和浊度,其中棉花秸秆灰的综合表现最优,对三者的降低幅度分别达56.71%、57.24%和77.37%。最后,用添加生物质灰后的沼液富CO_2溶液培养大白菜种子,其发芽指数整体大于0.8,表现出较低的植物生理毒性。因此,生物质灰可用于辅助沼液氮磷脱除,有利于沼液后期施用,其中,棉花秸秆灰的效果最优。

In order to reduce the operation cost of ammonia nitrogen and phosphorus removal from biogas slurry（ BS） through the ＂thermal-alkaline stripping ＂ process,biomass ash（ BA） addition was put forward to elevate the value of p H and precipitate the phosphorus in BS. Totally four types of BA were added into BS and pure water,including rice straw ash（ RS）,groundnut shell ash（ GDS）,cotton straw ash（ CTS） and corn straw ash（ CNS）. Main water qualities of those solutions,including chemical oxygen demand（ COD）,concentration of suspended solid（ SS）,total phosphorus（ TP） and total ammonia content（ TAN）,p H values,as well as calcium（ Ca） and magnesium（ Mg） contents were tested. The germination index（ GI） values of Chinese cabbage seeds treated by these biogas slurries after CO2 saturated were evaluated as well. Among those BAs,CTS had the highest solubility of about 12. 97%and GDS had the lowest value of 10. 67%. The results showed that when the BAs were added into the pure water,the p H value of water can reach 10. 0 ~ 12. 5. But when the same dosages of BAs were added into BS,the p H value of BS can only be elevated to 9. 5 ~ 11. 0 from 7. 87 due to the good buffering characteristics of BS. The main reason of p H value improvement of BS might be the precipitation of CO_2 in BS by Ca^（2＋）or Mg^（2＋）leached from BA. This relatively high p H value could meet the requirement of＂thermal-alkaline stripping＂process for ammonia nitrogen removal,which may contribute to reducing theoperation cost greatly. On the other hand,phosphorus removal efficiency was generally increased with the increase of BA dosage. The highest phosphorus removal efficiency of 78. 74% was acquired when100 g / L CTS was added into BS,but the efficiency was decreased inversely when BA dosage was increased to 200 g / L. The results also showed that when the BA dosage was increased to 200 g / L,the removal efficiency of COD and SS were above 50%. For the higher value of p H and phosphorus removal efficiency obtained by CTS addition,CTS was considered as the optimal type of BA. The germination index of Chinese cabbage seeds treated by CO_2-rich BS solutions after BA addition was generally higher than 0. 8,implying the low phytotoxicity. Therefore,the addition of BA into BS may be beneficial to ammonia nitrogen and phosphorus removal,and may not result in a rapid increase in phytotoxicity.