Reversing clogging in Imhoff tanks by catalyzed hydrogen peroxide treatment

Imhoff tanks (ITs) are a traditional sewage treatment system in common use in the areas lacking centralized sewage treatment facilities in Malaysia. The system consists of a tank filled with rocks as a filtering medium, over which sewage is applied intermittently. Micro-organisms attach to the media and form a biological layer. Organic matter in the sewage diffuses into the film, where it is metabolized by the micro-organisms. Oxygen is supplied to the biological layer from the air as the sewage diffuses though the IT. The method is a simple and economical, and has proved to remove soluble Biological Oxygen Demand (BOD) from sewage effectively.

One of the disadvantages of the system, however, is clogging. This can arise from several factors, including blockage between or on the surface of filter media by suspended solids particles from the raw sewage, blockage by micro-organism over-growth, and/or microbial metabolite deposit such as bacterial polysaccharide slimes on the filter media. These factors, together or separately, can inhibit treatment and cause hydraulic failure of the system. The traditional restoration procedure is to remove and replace the clogged media using clean material. This is costly and the facility might have to close for a time. Over the years a wide variety of chemicals has been used to try to deal with IT clogging, but no effective solution has been found for controlling its extent.

One important cleaning mechanism is to oxidize and destroy the accumulated solids and remove them. Several methods can be used for this including, oxidative, thermo-chemical and biological processes (Brooks & Grad (1968); Eastman & Ferguson (1981)).

Catalyzed hydrogen peroxide (CHP) comprises the use of hydrogen peroxide and a catalyst known as Fenton's reagent (Fenton 1894). It is often used to reduce organic loads or the toxicity of different wastewaters (Pignatello et al. (2006); Huang et al. (2001)).

In this study, the effectiveness of CHP on accumulated solid segregation and the efficacy of restoration of failed IT system using CHP were investigated.

Samples of accumulated solids used in this study were collected from an IT plant. The solids had Dry Solid content (DS) of 9,213 mg/L (0.92%DS). They were passed through a sieve to remove large debris and then thickened and adjusted to obtain initial total solid concentrations of 1.0%, 1.5% and 2.0%, respectively (DS). It was stored at 4 °C before use.

The study was designed to examine and evaluate the effects of CHP on accumulated solid segregation from bottom to water surface. The equipment consisted of 51 × 1 L measuring cylinders, as filter media tanks, into each of which a 1 L sample of the test solid was placed. All cylinders stood for 60 minutes before CHP treatment to ensure all solids accumulated at the bottom.

The CHP solution consisted of 598.06 g/L H₂O₂ and 0.42 g/L FeCl₃. The solution additions ranged from 0.5 to 8 mL to the cylinders containing the clogging solids. The experiment was repeated three times and the parameter average were used to assess the removal efficiency for the accumulated solids.

Solids concentration measurement was carried out according to APHA's TSS Method (American Public Health Association (APHA) 1995), before and after 30 minutes of CHP treatment.

Six filter media clogged IT systems in Klang Valley, referred to as STP-A, STP-B, STP-C, STP-D, STP-E and STP-F, were chosen for the study. Their filters were 112 m³, 134 m³, 523 m³, 765 m³, 306 m³ and 100 m³ volume, respectively. Clogging had reduced filter permeability in each case and the media were submerged in sewage. During peak flow, effluent was spilling over the tops of the media tanks.

The estimated CHP dosing rate was taken as 5 L-CHP/m³ of filter media and the solution was introduced into several places in each tank through a 75 mm pipe.

Water level measurement and the condition of flow from the media tank outlets were used to determine treatment effectiveness.

Laboratory scale study showed that, after CHP treatment, the sludge was effectively loosened and lifted from the bottom of each cylinder. Several minutes later, the reaction subsided and suspended particles gradually moved to the water surface (Figure 1). This was apparently caused by CHP penetration into the solid matrix and decomposition incorporating gas bubbles within the solid.

Figure 2 shows the effects of CHP dosage on the solid segregation rate. In concentration from 0.5 to 1 mL-CHP/L, the solid effervesced slowly and continuously, and its volume increased progressively. The effects were much greater at a dosing rate of 3 mL-CHP/L, and all of the solids moved to the surface at concentrations of 5 mL-CHP/L and above. For different DS concentrations the reactions differed. Measuring cylinders with 1.0% DS showed complete surface segregation at 5 mL-CHP/L. For 1.5% and 2.0% DS, doses of 6.5 mL-CHP/L and 7.5 mL-CHP/L were needed to achieve the same effect, respectively.

Figure 3 shows the ‘released’ solids floating on the top of the filter tank at STP-E. Several minutes after CHP treatment, the clogged solids began to effervesce and floated to the surface. CHP treatment produced remarkable segregation at the top of all six clogged tanks.

The mechanisms affecting CHP segregation of the solids in the tanks are similar to those observed in the laboratory experiment. It is thought that the CHP decomposes the solids clogging filter by oxidation. This causes bubble formation as the CHP reacts and the bubbles strip the solid materials from the filter media. This produces conditions under which the solid removed from the filter media float to the surface in the tank.

Figure 4 shows the condition of the filter tank before and after CHP treatment at STP-F. Before treatment, water filled the tank. After treatment, the solids had come to the surface and the effluent below began to flow out. All six clogged tanks produced same results.

Table 1 shows that the CHP dosages and restoration times of six clogged tanks were different. Clogging took between 2 and 96 hours to clear, and the use of CHP was between about 9 and 78% of that estimated. This was due to the different clogging conditions in the different tanks. The actual CHP dosage was decided on the basis of the amount needed to restore the clogged filter so that wastewater could pass through it naturally. Although the actual filter media could not be seen, the wastewater passed through the system smoothly and the effluent drained from the tanks naturally. This demonstrated that the clogging solids had been removed, which confirms that CHP can be used to restored clogged filter media.

This study demonstrated the efficacy of the removal of clogging solids and restoration of IT systems using CHP. The results show that

• Oxidation and segregation of accumulated solids in IT can be performed using CHP dosed at 5 mL/L.

• CHP is effective in restoring clogged IT systems.

Further research into the mechanisms of CHP on cleaning of filter media is required to confirm this finding for wide application.