Denis Couillard

The use of peat in wastewater treatment

Abstract Peat has been widely used in the treatment of wastewaters. The potential of using peat in wastewater treatment is reviewed with special attention to the following topics: (i) the properties of peat; (ii) the pretreatment of peat; (iii) the principles involved in the removal of wastewater pollutants by peat; and (iv) the applications of peat to the removal of impurities from wastewater.

Heavy metals removal from anaerobically digested sludge by chemical and microbiological methods

The metal removal from anaerobically digested sludge was studied by chemical treatment and microbial leaching processes in laboratory reactors. The removal of metals increased with decreased sludge solid concentration and pH. In the acid treatment process, a pH of 1.5 was required to reduce the copper concentration in the sludge to an acceptable level. In the microbial processes mixed culture gave 10% better solubilisation of metals than in single culture. The acid requirement was lower for the microbial process. The cost of sludge treatment in terms of chemicals was found to be decreased by 80% in microbial leaching.

Optimum residence time (in CSTR and airlift reactor) for bacterial leaching of metals from anaerobic sewage sludge

The bacterial process of metal solubilization from sewage sludges was studied using two types of reactors: a continuously stirred tank reactor (CSTR) and an airlift reactor, with partial sludge recycling in each case. Both reactors showed similar metal solubilization efficiencies. After parameter optimization and steady state were obtained, a mean hydraulic residence time (t) of 0.75 day allowed 91% Cu, 94% Zn, 93% Mn, 67% Ni and 67% Cd solubilization in the CSTR while 89% Cu, 91% Zn and 82% Cd were solubilized in the airlift reactor. The equation rv = 1.23 C + 0.44, expressing the solubilization rate of Cu or Zn according to their respective concentrations, had an explained variance (r2) of 0.93 for t = 0.75 d. This equation applies equally to the CSTR and to the airlift, thus showing the equality of the efficiency of either reactor. A significant first order kinetic equation has been formulated for each t. These results represent a 4:1 reduction of the required t in comparison to t used in earlier work and 0.75 day is therefore an optimum mean hydraulic residence time for bacterial leaching of metals from anaerobic sewage sludge.

Bacterial leaching of heavy metals from sewage sludge-bioreactors comparison.

More than 50% of municipal sewage sludges cannot be used on agricultural land because of their heavy metals content. Therefore, microbial leaching of heavy metal from municipal sludge was studied in a continuously stirred tank reactor without recycling (CSTR) or with sludge recycling (CSTRWR) at residence times of 1, 2, 3 and 4 days. The reactor CSTRWR is supposed to be more efficient for bacterial process due to the recycling of active bacteria from the settling tank to the reactor. The CSTRWR and the CSTR with 1 g litre(-1) FeSO(4).7H(2)O addition were equally efficient because of copper reprecipitation or recomplexation in the settling tank of the CSTRWR. In the CSTR, about 62% of copper and about 77% of zinc were dissolved in 3 days residence time compared to 50% of copper and 64% of zinc in the CSTRWR, if 3 g litre(-1) FeSO(4).7H(2)O was added. Thus with larger amount of substrate, the CSTR was more efficient than the CSTRWR. Residence time and pH were the main factors for zinc solubilization while for copper, the redox potential was also a major factor. The effect of FeSO(4).7H(2)O concentration on bacterial activity to solubilize heavy metals was also studied, increased concentration of FeSO(4).7H(2)O yielded better copper solubilization while it had no effect or a negative effect on zinc. This supports the hypothesis of a direct mechanism for zinc solubilization and of an indirect mechanism for copper solubilization.

Slaughterhouse effluent treatment by thermophilic aerobic process

A pig slaughterhouse effluent was submitted to laboratory runs in order to assess the potentiality of the thermophilic aerobic process for the treatment of meat processing industrys effluents. A mixed aerobic bacterial culture was successfully maintained at 52 and 58°C in a semi-continuously fed bioreactor, without recycling the cells. Operation was conducted at 6, 12, 18, 24 and 30 h of solids retention time. Over 93% of the CODs (Chemical Oxygen Demand) were removed during the treatment at 52°C for all the retention times investigated, as compared to 86% of removal at 58°C, with an exception for the case of 6 h retention time. Reduction of phosphorus in the form of orthophosphate ranged between 72–90% with the best efficiency noted at 52°C and 6 h retention time. Extremely high specific utilization rates (qm) were observed and revealed that the process is about 10 times faster than the mesophilic process for slaughterhouse effluent treatment. Biokinetics parameters μm (maximal specific growth rate) and kd (endogenous respiration coefficient) were evaluated by fitting to appropriate mathematical models. They were found superior to those reported in the literature for mesophilic process, with the exception of the Y (actual yield) coefficient which is of the same magnitude. High kd values had a marked effect on the apparent yield of sludges which showed low values as compared to those found in mesophilic systems.

Essential interactions between Thiobacillus ferrooxidans and heterotrophic microorganisms during a wastewater sludge bioleaching process.

The stimulating effect of heterotrophic microorganisms was investigated on the growth and on the ferrous iron oxidation of Thiobacillus ferrooxidans in synthetic media and in wastewater sludge. The addition of a sediment. Rhodotorula rubra isolate or a strain of T. acidophilus on two-layer agarose-gelled medium doubled the plating efficiency of T. ferrooxidans. In liquid cultures, R. rubra had a slight but significant effect on the growth rate of T. ferrooxidans. Moreover, the yeast allowed a faster initiation of the ferrous iron oxidation and acidification by T. ferrooxidans. In the bioleaching process, the co-culture of T. ferrooxidans with R. rubra or with the indigenous microbial assemblage from sludge was shown to be essential since the pure culture of T. ferrooxidans failed to oxidize ferrous iron and to acidify wastewater sludge. These results emphasize the importance of active heterotrophic microorganisms in the metal bioleaching activity of T. ferrooxidans in sludge.

Thermophilic aerobic process for the treatment of slaughterhouse effluents with protein recovery.

A pig slaughterhouse effluent was submitted to laboratory runs in order to assess the potential of the thermophilic aerobic process for the treatment of slaughterhouse and meat processing industry effluents with protein recovery. Experiments were conducted at 45 degrees C, 52 degrees C and 58 degrees C with solids retention times of 6, 12, 18, 24 and 30 h in a semi-continuously fed bioreactor. At 45 degrees C and 52 degrees C, over 90% of the Chemical Oxygen Demand (COD) was removed at all retention times investigated. At 58 degrees C, COD decreased by 86%, with the exception of the 6-h retention time. A biomass containing 70% Gross Protein (GP) was recovered. The optimal specific productivity of GP (0.434 mgGP mgCOD(-1) h(-1)) occurred at 52 degrees C with a 6-h retention time. Biokinetic parameters were evaluated by fitting to appropriate mathematical models. Analysis of the GP composition confirmed that the amino acid levels in the recovered biomass meet most animal requirements.

Removal of metals and fate of N and P in the bacterial leaching of aerobically digested sewage sludge

Abstract The solubilization of metals from aerobically digested sewage sludge was performed biologically using Thiobacillus ferrooxidans, in a continuously stirred tank reactor (CSTR). Experiments involved the preacidification of the sludge to pH 4.0 and the addition of 4 g FeSO4-7H2O per liter of sludge. Reactors were operated with 20 g/l total solids (2% TS), 0.5 volume of air per reactor volume per minute, agitation at 300 rpm and a 20% recirculation rate. A minimal mean hydraulic residence time (t) of 0.75 day, one half the time required in a batch system, resulted in solubilization of 52% of the Cu, 62% of the Zn and 78% of the Mn. The leached sludge was more easily dewatered than untreated sludge, and would not likely require the addition of polymer if a plate filter press were used. Metals did not reprecipitate during filtration. A small quantity of added lime was sufficient to precipitate the metals; selective precipitation could permit the separation of ferric hydroxide from valuable metals (Cu, Zn, Ni, Cd), which may then be recovered. No significant changes in the total nitrogen and phosphorus content occurred during solubilization. Biological solubilization produced a better quality sludge for agriculture, as well as avoiding the dispersal of metals into the environment.

Strategies to maximize the microbial leaching of lead from metal-contaminated aquatic sediments

Different strategies designed to increase the removal of Pb were tested during the application of a biological remediation procedure to treat highly-contaminated aquatic sediments. The use of FeCl2 instead of FeSO4·7H2O as a substrate for thiobacilli bacteria did not interfere with the biological solubilization process which occurred in sediments acidified to pH 4 with H2SO4. With FeCl2, twice as much Pb was solubilized (5 mg Pb l−1) compared to the same strain acclimated to FeSO4·7H2O. With FeCl2, the solubilization of other trace metals (Cu,Zn) was similar to that obtained with the strain acclimated to FeSO4·7H2O. If the sediments were acidified with HCl, rather than H2SO4, then the solubilization of Pb increased to 11 mg Pb l−1: this was five times greater than when a strain acclimated to FeSO4·7H2O was used in sediments acidified with H2SO4. Theoretical calculations with MINEQL+ (chemical equilibrium program) predicted 10.8 mg Pb l−1 under similar conditions. This biological solubilization process was generally poorly adaptable to changes in the source of acidity (i.e. HCl vs H2SO4). The solubilization of Cu remained relatively low (solubilization of only 35% after 72 h) since the inadequately low levels of sulphate prevented the Thiobacillus bacteria from properly developing. Finally, the higher concentration of chloride ions gained through an increased level of total solids (7% rather than 3% TS) did not increase the soluble concentration of Pb. The soluble concentration of Pb remained at 11 mg Pb l−1 (46%), whereas theory predicted a concentration of 24.1 (100%) mg Pb l−1. Most of the experimental results seemed to follow the theoretically predicted outcome for Pb solubilization. There is more work to do to optimize the process. However, this work reveals that it seems possible to develop an economical process for the removal of Pb from contaminated sediments.

Bacterial leaching of heavy metals from sewage sludge for agricultural application

Bacterial leaching of heavy metals from sewage sludge was studied to assess the potential of agricultural application of the decontaminated sludge. Experiments were conducted in both batch processes (at 28 ‡C) and continuous process (at 30 ‡C) using a culture of Thiobacillus ferrooxidans and a mixted culture of Thiobacillus ferrooxidans and Thiobacillus thiooxidans. Reagent grade ferrous sulfate, spent ferrous sulfate and pyrite were used as the energy substrates. A first order kinetic model of Cu solubilization was proposed. It was shown that the processes in the continuously stirred tank reactor (CSTR) and the air lift reactor (ALR) with 20% cell recycling were more efficient than the others for metal solubilization due to a relatively large bacterial population.