Jiro Aizawa

Removal of heavy metals from anaerobically digested sewage sludge by a new chemical method using ferric sulfate

The removal of heavy metals from anaerobically digested sewage sludge was studied by using ferric sulfate. The addition of ferric sulfate to the sludge caused the acidification of the sludge and the elution of heavy metals from the sludge. The pH of the sludge decreased with an increase in the amount of iron added and with a decrease in the sludge concentration. At a sludge solid concentration of 2% (w/w), the sludge pH dropped below 3 and the elution percentage of cadmium, copper and zinc was more than 80% when the added amount of ferric iron was more than 1.5 g per L of wet sludge. Furthermore, the method using ferric sulfate was compared with that using sulfuric acid at pH 3 in order to clarify the effect of ferric iron as an oxidation reagent on elution of heavy metals. Ferric iron eluted cadmium, copper and zinc more effectively than sulfuric acid. This effective elution of heavy metals was caused by the oxidation of the sludge solid by ferric iron added. From these results, it was concluded that ferric iron played a role to acidify the sludge and to oxidize metallic compounds in the sludge and this new chemical method was useful for the removal of heavy metals from anaerobically digested sewage sludge.

Biotransformation of arsenic species by activated sludge and removal of bio-oxidised arsenate from wastewater by coagulation with ferric chloride.

The potential of activated sludge to catalyse bio-oxidation of arsenite [As(III)] to arsenate [As(V)] and bio-reduction of As(V) to As(III) was investigated. In batch experiments (pH 7, 25 degrees C) using activated sludge taken from a treatment plant receiving municipal wastewater non-contaminated with As, As(III) and As(V) were rapidly biotransformed to As(V) under aerobic condition and As(III) under anaerobic one without acclimatisation, respectively. Sub-culture of the activated sludge using a minimal liquid medium containing 100mg As(III)/L and no organic carbon source showed that aerobic arsenic-resistant bacteria were present in the activated sludge and one of the isolated bacteria was able to chemoautotrophically oxidise As(III) to As(V). Analysis of arsenic species in a full-scale oxidation ditch plant receiving As-contaminated wastewater revealed that both As(III) and As(V) were present in the influent, As(III) was almost completely oxidised to As(V) after supply of oxygen by the aerator in the oxidation ditch, As(V) oxidised was reduced to As(III) in the anaerobic zone in the ditch and in the return sludge pipe, and As(V) was the dominant species in the effluent. Furthermore, co-precipitation of As(V) bio-oxidised by activated sludge in the plant with ferric hydroxide was assessed by jar tests. It was shown that the addition of ferric chloride to mixed liquor as well as effluent achieved high removal efficiencies (>95%) of As and could decrease the residual total As concentrations in the supernatant from about 200 microg/L to less than 5 microg/L. It was concluded that a treatment process combining bio-oxidation with activated sludge and coagulation with ferric chloride could be applied as an alternative technology to treat As-contaminated wastewater.

Enhanced heavy metals removal without phosphorus loss from anaerobically digested sewage sludge

Heavy metals removal without phosphorus loss from anaerobically digested sewage sludge was investigated by conducting batch experiments using hydrogen peroxide and/or iron sulphate under acidified conditions at pH 3. The addition of hydrogen peroxide to the sludge improved the elution efficiencies of As, Cd, Cu and Zn with phosphorus loss from the sludge. The optimum initial concentrations of hydrogen peroxide were. Respectively. 0.1% for As, Cd, Mn and Zn and 0.5% for Cu and Ni. The combined process of 0.1% hydrogen peroxide and 1 g Fe/L ferric sulphate enhanced the initial elution rate of Cu and Cr compared to the addition of either ferric sulphate or hydrogen peroxide, indicating that oxidants stronger than hydrogen peroxide were produced in the sludge. Furthermore, the combined process immobilised phosphorus in the sludge due to co-precipitation with ferric hydroxide or precipitation as ferric phosphate. It was concluded that there is a possibility that the combined process could remove heavy metals effectively without phosphorus loss from anaerobically digested sewage sludge.

BIOLOGICAL FERROUS-IRON OXIDATION WITH FLUIDIZED BED REACTOR

ABSTRACT The kinetics of ferrous iron oxidation by iron oxidizing bacterium Thiobacillus ferooxidans was studied in batch experiments. The effects of dissolved oxygen and initial ferrous iron concentration on the rate of ferrous iron oxidation could be expressed by Monod-type equations. The maximum specific oxidation rate was 3.6 × 10 −9 mg/h/cell and saturation constants with respect to ferrous iron and dissolved oxygen were 140 mg/1 and 0.162 mg/1, respectively. Among the support materials studied, the macroporous ion exchange resin Amberlite IRA-938 had the best attachment of iron oxidizing bacteria. By use of this resin as a support medium, continuous experiments with a fluidized bed reactor were carried out to study a biological oxidation of ferrous iron. An over 90% efficiency of ferrous iron oxidation was obtained at influent ferrous iron concentrations of 0.5 and 1.0 g/1 within a range of hydraulic retention times of 0.4–2.5 hrs. On the basis of these experimental results, mathematical models were developed to describe the behaviour of ferrous iron concentrations in the support media and in the fluidized bed reactor. These models well expressed ferrous iron concentrations in the effluent.