硝酸鹽濃度對小球藻生長期間衍生消毒副產物前驅物特性之影響

Abstract

由於過量的硝酸鹽、磷酸鹽排入水庫，提供藻類生長足夠的營養源，使水庫發生優養化現象，造成水庫水質惡化，原水含有高濃度之藻類有機物會增加淨水場處理單元操作的困難。有鑑於藻類有機物中的碳水化合物、胺基酸及含蛋白質成分的有機物是產生消毒副產物的重要前驅物質之一，因此本研究以綠藻中之小球藻為對象，著重在硝酸鹽進流濃度如何影響綠藻生長行為，以及在各生長率下所產生的藻類有機物特性變化，從而使加氯消毒後含碳消毒副產物生成潛能(三鹵甲烷與鹵化乙酸)不同而進行研究。本研究以高、低硝酸鹽進流濃度條件(1及 0.1 g/day KNO3)下在光生物反應器內分別培養小球藻，並以葉綠素a、藻數、乾重做為生長變化參數，在不同生長率下分別對小球藻產生的藻類有機物進行分離，以冷凍乾燥-研磨-加熱法分別分為胞外有機物、胞內有機物及細胞有機物，螢光特性及分子量分布分別使用螢光激發/發散陣列光譜儀及高效能粒徑排除層析儀進行分析。根據3D螢光光譜結果，不同生長率下胞外有機物組成成分以類黃酸物質為主，胞內有機物與細胞有機物特性相似且以類芳香族蛋白為主；藻類有機物分子量分布大部分都集中在小分子物質，且當藻類生長率小於零時，小分子有機物比例增加。此外本研究發現在高、低硝酸鹽進流濃度條件下所培養的小球藻藻類有機物其鹵化乙酸生成潛能均大於三鹵甲烷生成潛能，且所占百分比高達70%左右，由此可知藻類有機物內含鹵化乙酸前驅物多於三鹵甲烷；在高硝酸鹽進流濃度條件下培養各生長率下的藻類有機物的C-DBPFP幾乎都高於低硝酸鹽進流濃度條件下培養正生長率之藻類有機物的C-DBPFP，本研究證實硝酸鹽進流濃度提高會使小球藻藻體的直徑增長、降低藻類有機物的C/N比進而增加其生成的C-DBPFP，由皮爾森雙變數相關性分析的結果證明小球藻藻類有機物中的類芳香族蛋白與類微生物副產物是與氯反應生成HAAs之主要前驅物，藻類有機物中醯胺類(N-C)的物質是主要生成C-DBP的化學官能基。

Algal blooming occurs at the excess nitrate and phosphate in reservoirs because these matters would provide algae with enough nutrients. At such a condition, a large number of algogenic organic matter (AOM) derived from algae will worsen the quality of raw water, which causes increasing difficulty in operating the unit of water treatment plants. The AOM contains a great numbers of carbohydrate, amino acids and proteinaceous compounds that are potential disinfection by-product precursors. This study aimed to investigate the effects of nitrate concentration on a green algae Chlorella sp. growth behavior in cultivation as well as the AOM characteristics under different growth rates and its corresponding effects on the formation potential of carbonaceous disinfection by-products (C-DBPs), including haloacetic acids (HAAs) and trihalomethanes (THMs). In this study, the Chlorella sp. was cultured at high and low nitrate concentration (i.e., 1 and 0.1 g/day KNO3) in photo-bioreactor. The algal growth rate was determined using various indexes such as chlorophyll a, cell number, dry mass. Those parameters were used to evaluate algal growth behavior. The fractions of AOM was divided into three parts, including extra- (EOM), intra- (IOM) and cell-bond organic matter (COM) by freezing-grinding-heating protocol. Fluorescent properties and molecular weight (MW) distribution of AOM were determined using fluorescent extraction-emission matrix (FEEM) method and high performance size exclusion chromatography (HPSEC), respectively. The results have shown that EOM predominantly constitutes of fulvic-like substances, while IOM mainly constitutes of aromatic protein-like substances and COM has a similar fluorescent composition to IOM. On the other hand, the EOM, IOM and COM predominantly compose organic substances with small MW (< 1KDa). These compounds with small MW (< 1KDa) of AOM slightly increase when algae growth rates decline from positive to negative. In addition, AOM derived by Chlorella sp. has high amount of HAAFP, namely 70% of total the C-DBPFP, more than that of THMFP. The increase in nitrate concentration would enlarge the average diameter of Chlorella sp. and lower C/N ratio. It produces larger amounts of THMFP and HAAFP when AOM derived from algae cultured at high nitrate concentration. The results of Pearson correlation coefficient analysis proved that aromatic protein-like and SMP-like substances are the major precursors to the formation of HAA as well as functional group of amide (N-C) plays a major role in the formation of C-DBP.