Two biological contact oxidation reactors, cascade biofilm reactor (CSBR) and one-step biofilm reactor (OSBR), were used in this paper for pretreatment of eutrophic water from Lake Taihu in China. The CSBR was more effective and stable for eutrophic water treatment than OSBR, in terms of extracellular microcystin-LR,
A large amount of nitrogen, phosphorous, and other components can be entered into natural lake water due to the discharge of pollutants into a water body. The excess existence of these elements will lead to lake eutrophication, algal blooms, and massive accumulation of harmful organic micropollutants. Lake Taihu, the third-largest freshwater lake in China, has received increasing amounts of waste from industrial, agricultural, and municipal development, as a result of rapid economic development and related urbanization along the lake. As Wang et al. indicated, two hundred and seventy-three kinds of organic chemicals in water from Taihu Lake were examined, 200 more than those detected in 1985. Among them 21 kinds of chemicals belong to priority pollutants and 17 kinds are the endocrine disruptors. The number of organic pollutants recorded in their tests is 3.6 times that determined in 1985, while concentrations of some pollutants were more than 2 times higher than in 1985 [
Organic pollutants in eutrophic water mainly include algal toxins, pesticides, persistent organic pollutants (POPs) such as polychlorinated biphenyls (PCBs), dioxins, and furans; endocrine disruptors (EDs) such as polycyclic aromatic hydrocarbons (PAHs) and phthalate esters (PAEs); and humic acids, which are precursors of disinfection by-products. Many organic pollutants have toxic effects on the human liver, kidneys, immune system, endocrine system, and reproductive system. Moreover, these organic pollutants might have the potential to cause abnormalities, cancer, and mutation. In the Meiliang Bay area of Lake Taihu, the average levels of microcystins (MCs) could reach as high as 10~18
The conventional drinking water treatment process, comprised of coagulation, sedimentation, sand filtration, and disinfection stages, is inefficient in removing DOC, with removal efficiencies amounting to only 20%~30% [
The objective of this study was to investigate the biological pretreatment of organic pollutants in the eutrophic Lake Taihu raw water, for the enhanced treatment, safety, and quality of drinking water. Two biological contact oxidation reactors, cascade biofilm reactor (CSBR) and one-step biofilm reactor (OSBR), were adopted in this research.
The pilot test was conducted in Wuxi Water Works, in Nanquan Town, China, near the north side of Lake Taihu.
The CSBR was composed of 3 cells (Figure
The schematic diagram of two biological pretreatment reactors. CSBR: cascade biofilm reactor; OSBR: one-step biofilm reactor. 1. Inlet pipe, 2. outlet pipe, 3. air inlet pipe, 4. mud pipe, 5. effluent flume, 6. filler, and 7. water and air arrangement plate. Reactor size: the OSBR is square: length × width = 600 × 600 mm; the cells of CSBR are round; the diameter of each cell is 380 mm.
The raw water from Lake Taihu was pumped to the individual reactors. The hydraulic retention time of both reactors was 2 h. The gas-water ratio was 1.5 : 1 for each reactor. Samples were collected when the reactors were both in steady state operation.
The basic water quality parameters of raw water from Lake Taihu were summarized as follows: water temperature was from 5.5°C to 13°C, turbidity was from 6 NTU to 20 NTU,
A guide sample of microcystin-LR was purchased from Sigma (USA). Guide samples of atrazine, three phthalic acid esters (dimethyl phthalate (DMP), di-(2-ethylhexyl) phthalate (DEHP), and di-n-butyl phthalate (DBP)), and 16 US EPA priority PAHs (naphthalene (Naph), acenaphthylene (Aceph), acenaphthene (Ace), fluorine (Fl), phenanthrene (Phe), anthracene (An), fluoranthene (Flu), pyrene (Pyr), benzo[a]anthracene (BaA), chrysene (Chr), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), benzo[a]pyrene (BaP), indeno[1,2,3-cd]pyrene (Inp), dibenzo[a,h]anthracene (DBA), and benzo[ghi]perylene (BgP)) in a mixture solution of 2000
DOC was detected using the TOC automeasuring device TOC-
Atrazine, three phthalic acid esters (PAEs) species, and polycyclic aromatic hydrocarbons (PAHs) were all determined by gas chromatography (Shimadzu, GC-2001, Japan). First, a 1 L water sample was enriched by C18 solid phase extraction column. In the second stage, the C18 solid phase extraction column was then eluted using methanol, ethyl acetate, and hexane solution containing 5% of isopropanol for eluent. Third, the eluent was concentrated and the final volume was diluted to 1.0 mL. Finally, the concentrations of atrazine, PAEs, and PAHs were detected using gas chromatography. The electron capture detector (ECD) was used for atrazine and PAEs while the flame ionization detector (FID) was used for PAHs.
The enrichment of algal biomass in eutrophic water was accompanied by
Removal of
The removal of algae by biological pretreatment may occur through the following mechanisms: adsorption by biofilm; microbial oxidation and decomposition; mechanical retention and biological flocculation by filler; prey by microanimals; biological flocculation; and algal sedimentation by biofilm off the filler. Because of their small size, it is difficult for blue-green algae to be removed by mechanical retention. Therefore algae removal should mainly rely on biofilm adsorption, biological flocculation and sedimentation, and the role of microanimals.
Microcystins, mainly produced by freshwater blue-green algae, are the most common algal toxins. They have the highest occurrence frequency in cyanobacteria blooms and have already resulted in very serious impacts on people [
Removal of microcystin-LR by two reactors.
Figure
Removal of dissolved organic matter (DOC) and biodegradable dissolved organic matter (BDOC) by two reactors.
Biodegradable DOC (BDOC) refers to soluble organic compounds in water which can be decomposed by bacteria to CO2 and water or to other materials used for cell metabolism [
Atrazine is one of the most widely used herbicides in the world and has had a wide range of applications in China since the 1980s. Atrazine is classified as a suspicious substance because of the presence of environmental hormones (endocrine disrupters) and is considered by the United Nations to be one of 27 persistent toxic substances, due to its large consumption, long residual period (a half-life of 244 days), and possible carcinogenic effects in human and mammals [
Typical conventional processes, such as coagulation and sedimentation, lime softening, and chlorination, are not effective in the removal of atrazine and other herbicides [
The background concentration of atrazine in Lake Taihu was 136.5 ng/L. The CSBR could reach a satisfactory removal efficiency for atrazine (about 57.5%, Figure
Removal of atrazine by cascade biofilm reactor.
Phthalic acid esters (PAEs), a group of important organic chemicals, have a wide range of applications in many industries and have become one of the most common pollutants in the world. Studies on the reproductive toxicity of PAEs show that these substances have endocrine effects on organisms and can cause proliferation of cancer cells. The toxicity may even have intergenerational effects through the placenta and through breastfeeding [
Three kinds of PAEs (DMP, DEHP, and DBP) were selected for the investigation of removal characteristics via biocontact oxidation pretreatment. The background concentrations of DMP, DEHP, and DBP were 5.61
Removal of three kinds of PAEs by cascade biofilm reactor.
PAHs are widely distributed in the atmosphere, water, mud, and soil and are the result of
The background concentration of total PAHs in Lake Taihu was 21.51
The removal rate of PAHs by cascade biofilm reactor (CSBR)/
Name | Rings | Raw water | CSBR-1 | CSBR-2 | CSBR-3 |
---|---|---|---|---|---|
Naphthalene | 2 | 1.084 | 1.396 | 1.597 | 1.395 |
Acenaphthene | 3 | 1.746 | 0.907 | 1.415 | 0.870 |
Acenaphthylene | 3 | 2.164 | 5.171 | 3.694 | 3.331 |
Fluorene | 3 | 0.105 | 0.069 | 0.072 | 0.098 |
Phenanthrene | 3 | 0.108 | 0.064 | 0.065 | 0.063 |
Anthracene | 3 | 0.533 | 0.264 | 0.261 | 0.275 |
Fluoranthene | 4 | 1.130 | 0.303 | 0.283 | 0.360 |
Pyrene | 4 | 0.720 | 0.179 | 0.944 | 0.142 |
Benzo(a)anthracene | 4 | 2.907 | 1.644 | 1.919 | 2.648 |
Chrysene | 4 | ND | ND | ND | ND |
Benzo(b)fluoranthene | 5 | 2.495 | 1.722 | 0.93 | 1.461 |
Benzo(k)fluoranthene | 5 | 2.960 | 0.090 | 0.167 | 0.168 |
Benzo(a)pyrene | 5 | 0.990 | 2.472 | 1.816 | 0.923 |
Indeno(1,2,3-cd)pyrene | 6 | 3.082 | 1.028 | 1.486 | 1.793 |
Dibenzo(a,h)anthracene | 5 | 1.314 | 0.146 | 0.033 | 0.148 |
Benzo(g,h,i)perylene | 6 | 0.169 | 0.480 | 0.104 | 0.191 |
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Total amount of PAHs ( |
21.507 | 15.935 | 14.786 | 13.866 | |
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Removal rate (%) | 25.6 | 30.9 | 35.2 |
Note: PAHs: polycyclic aromatic hydrocarbons. ND means the substance was not detected.
Biological pretreatment showed a relatively low removal efficiency (35.2%) for PAHs compared to other organic micropollutants (Table
The CSBR was more effective and stable than the OSBR for the treatment of eutrophic water, in terms of the removal efficiency of extracellular microcystin-LR, In CSBR, the removal of BDOC and DOC was mainly attributed to biodegradation. 11.5% of DOC was removed in ways other than biological degradation, such as through biofilm adsorption and bioflocculation. The CSBR could effectively promote atrazine and PAEs removal. 57.5% of atrazine was removed at 2 h HRT with a background concentration of 136.5 ng/L. The removal efficiencies of three PAEs were 78.7%, 52.4%, and 85.3%, respectively. Regarding PAHs, only 35.2% could be removed by the CSBR with an initial PAHs concentration of 21.5 The CSBR was more effective in pretreatment of low-molecular-weight organic pollution, and this reactor benefitted the improvement of effluent quality regarding drinking water treatment process.
The authors declare that they have no competing interests.
This work was financially supported by the National Major Program of Science and Technology for Water Pollution Control and Restoration of China (2014ZX07405002 and 2012ZX07403-001).