Final accepted version
Chemosphere 77 (2009) 279–284 http://dx.doi.org/10.1016/j.chemosphere.2009.07.036
Effect of humate on biological treatment of wastewater containing heavy metals Ewa Lipczynska-Kochanya and Jan Kochanyb a
b
Environmental Consultant, 30 Elm Dr. East, Suite 918, Mississauga, ON, Canada, L5A 4C3 Conestoga-Rovers & Associates, 111 Brunel Rd, Suite 200, Mississauga, ON, Canada L4Z 1X3
Abstract This paper presents results of investigations on the influence of humic substances (humate, HS) on the biological treatment of wastewater containing heavy metals (Cr, Cu, Fe, Mn, Ni, and Zn). Respirometric studies indicated that the investigated system complied with the Haldane model for inhibitory wastes. Chemical analyses showed that, while the SCOD removal was high (~82%), -1
only ~7% of ammonia was oxidized to nitrate. An addition of HS (500 mg L ) mitigated the inhibitory effect of the wastewater on the returned activated sludge (RAS). The system with HS complied with the Monod model for non-inhibitory wastes, and the removal of ammonia and metals was ~99% and over 90%, respectively. It is suggested that an application of HS could be beneficial for treatment plants receiving wastewater streams containing heavy metals.
Keywords: Wastewater treatment; Heavy metals; Respirometry; Humate; Nitrification
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Introduction
Heavy metals enter wastewater from a variety of domestic and industrial sources. Despite limits for metals in industrial discharges, their concentration in the sewage system may be significant if many such discharges are located in the same area. Metals exist in wastewater in a soluble and particulate form. During primary sedimentation of wastewater, heavy metals associated with the settleable fraction are usually removed. The removal during primary sedimentation depends on the metals properties and their interactions with various components of water (Kempton et al., 1987). While 40-70% of cadmium, chromium, copper and lead is typically removed, the removal of nickel and manganese is significantly lower (20-30%). Most of soluble metals are removed with wasted activated sludge (WAS) by a number of mechanisms including adsorption and complexation (Hu et al., 2003). Metals can bind to bacterial extracellular polymer, accumulate in the cytoplasm, or adsorb onto the cell wall. Nonessential 2+
2+
2+
metals, like Ni or Zn , may enter the cell and displace essential metals (e.g. Fe ) leading to the inhibition of physiological functions. Interactions of metals with intracellular functional (thiol) groups are believed to destroy protein structures. As hydraulic retention time (HRT) is substantially lower as compared to the sludge retention time (SRT), the content of metals in the activated sludge is several magnitudes higher than in the treated wastewater (Karvelas et al., 2003). Since heavy metals accumulate in the activated sludge they can reach inhibitory concentrations, particularly towards nitrifies, autotrophic bacteria involved in the process of nitrification (Hu et al., 2002, 2003, 2004; Principi et al., 2006).
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Nitrification is generally considered the controlling step in the biological nitrogen removal, and its inhibition by metals has attracted a considerable attention (Hu et al., 2002, 2003, 2004; Juliastuti et al., 2003; Park and Ely, 2008). Methods used to determine the inhibitory effects of various substances on the activated sludge include an application of respirometry as well as measurements of the nitrate and ammonia concentrations. Data obtained from laboratory tests are difficult to be applied in a wastewater treatment plant because wastewaters are complex combinations of often-unknown substances with interactive effects. It has been suggested that during an assessment of the wastewater toxicity, combinations of tests would be more reliable than only one particular test (Dalzell et al., 2002; Juliuastuti et al., 2003; Ren, 2004; Pagga at al., 2006). While inhibitory effects of heavy metals on the operation of biological treatment processes are well-documented (Stephenson and Lester, 1987; Karvelas et al., 2003), mitigation of these effects has received less attention (Hu et al., 2002, 2003). Humic substances (HS) are naturally occurring compounds resulting from microbial and chemical transformation of organic debris (Davis and Ghabbour, 1998). Their potential for the removal of metals from soil and groundwater has been extensively explored (Davis et al., 1997). However, the application in the wastewater treatment has received less attention. Iron humate, a complex metallo-organic material, is recommended for the removal of heavy metals from water (Janos et al., 2004). It has been recently reported that HS significantly accelerate degradation of inorganic and organic pollutants by the Fenton reaction at neutral pH (Lipczynska-Kochany and Kochany, 2008a) and they can also mitigate the inhibition of the activated sludge caused by high concentrations of organic substances (Lipczynska-Kochany and Kochany, 2008b).
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This paper describes treatability studies conducted on wastewater containing a mixture of heavy metals (Cr, Cu, Fe, Mn, Ni and Zn). The aim of this work was to explore a possibility of the practical application of HS in a biological treatment plant as an inexpensive method allowing reducing the content of ammonia and metals before the discharge to natural water bodies. Effect of HS on the returned activated sludge (RAS) in wastewater was investigated using aerobic respirometry. The HS impact on the nitrification and metals removal is also described.
2. Materials and methods
2.1. Materials Wastewater and RAS were collected from the same municipal treatment plant. RAS (total
suspended solids TSS = 3800110 mg L-1, volatile suspended solids VSS = 2900100 mg L-1) was acclimatized before tests for 6 days. Parameters of the wastewater were pH = 7.5±0.3, TSS = 42±2 mg L-1, VSS = 33±2 mg L-1, total phosphorus = 3.2±0.06 mg L-1 and alkalinity = 260±5 mg CaCO3 L-1. Soluble chemical oxygen demand (SCOD) as well as concentrations of metals, NH3N, and NO3-N is given in Table 1. Dilution water was prepared according to the standard procedure for BOD tests (Standard Methods, 1989). Chemicals (phosphate buffer, NH4Cl, MgSO4, CaCl2 and FeCl3) were from Anachemia Science, Lanchine, QC. HS was leonardite from New Mexico, used as a powder average size 15 µm. According to the supplier (U Mate Int., Scottsdale, AZ), it contained ~20% of fulvic acid (FA) and ~50% of humic acid (HA), determined by IHSS method (Swift, 1996). Organic carbon content was ~30%, total nitrogen –1.2%, Fe –1.1%, Al –1.2%, Mg – 0.2%, Na – 0.15%, and K– 0.04% respectively. Oxygen was medical grade from Praxair, Mississauga, ON.
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Table 1 Analyses of the influent (wastewater) and the effluents after the treatment with RAS without HS -1
and with 500 mg L of HS. Parameter
Effluent (with HS, 500 mg L-1)
Influent
Effluent (no HS)
(wastewater)
(mg L-1)
removed (%)
(mg L-1)
removed (%)
SCOD
168±1
301
82
461
73
NH3-N
230.5
15.00.5
35
0.240.05
99
NO3-N
500 mg L ) mitigated this inhibitory effect and the system complied with the Monod model for non-inhibitory wastes. The removal of ammonia and metals was ~99% and over 91%, respectively. The results suggest that an application of humic substances in wastewater treatment may be beneficial for wastewater treatment plants receiving wastewater streams with heavy metals.
Acknowledgements Authors wish to thank Mr. Wayne Smith for help with respirometric experiments and Dr. Alan Weston for his comments. The Referees’ suggestions are appreciated.
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_______________________________________________________________ Contact: E. Lipczynska-Kochany –
[email protected] J. Kochany –
[email protected]
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