DSD INTERNATIONAL CONFERENCE 2014 (DSDIC 2014) Ozone to Control Bulking and Foaming in Municipal Waste Water Treatment Plants

Ozone is known to be effective for achieving positive treatment effects on sludge characteristics. One of these effects is the reduction in excess sludge generation in wastewater treatment operations. This application is known for many years and full scale plants are installed and in operation. The economical success of this application strongly depends on the required ozone dose to reduce a certain amount of excess sludge expressed as kg O3/kg TSremoved. Economic dosage rates below ≤ 0.1 kg O3/kg TSremoved are proven in full scale plants. In addition a significant reduction of bulking at the wastewater treatment operation as a result of the ozonation process is observed. Microbiological analyses indicate that this is as a result of the greater vulnerability of filamentous bacterial species to sludge ozonation. Investigations show a significant reduction in the population of all filamentous microbial species, with the most significant reductions being observed in Microthrix parvicella and Nocardia species. The dosage range of ozone required enabling foam reduction and bulking control is significantly lower than the dosage range for the excess sludge reduction. Ozone is an attractive alternative to chemicals to prevent the sludge bulking and improves in parallel the settlement behaviour of the sludge (reduced Sludge Volume Index SVI). Depending on operational conditions the SVI can be readily reduced from 300 to lower than 100 ml/g. 1. OZONATION OF RAS (RETURN ACTIVATED SLUDGE) The most applied sludge ozonation processes are integrated in the return activated sludge line to reduce the growth of biomass and therefore to reduce the amount of waste activated sludge mass. Integration in RAS line means that a portion of activated sludge is treated by ozone in a bypass system and RAS is pumped back to the biological treatment step. Most common is the mixing of ozone and RAS with a venturi pipe. Some processes use a pressurized reactions system or a loop reactor to have a certain retention time. The following scheme shows the implementation on RAS treatment: Figure 1: Principle scheme for Sludge removal process by ozone 2. BULKING AND FOAMING CONTROL ON ACTIVATED SLUDGE SYSTEMS The effectiveness of solid-liquid separation in wastewater treatment operations can be affected by bulking and foaming [8, 12]. Bulking and foaming problems can be pervasive, affecting up to 50% of biological wastewater treatment facilities [8]. It has always been recognized that poor settleability and bulking is associated with the presence of filamentous organisms. A wide variety of filamentous organisms such as Microthrix parvicella, Eikelboom types 0041, 021N, 0092, 0675, Thiothrix, and norcadiaforms actinomycetes have been found in wastewater treatment systems. Microthrix parvicella is extremely pervasive. In a survey of 167 plants in Italy [8] found Microthrix parvicella to be the most common filamentous organism, and its presence was highly correlated with the incidence of both bulking and foaming problems. A variety of factors such as low DO, low F/M, complete mix reactor conditions, septic wastewater and sulfide availability, nutrient deficiency and low pH have all been associated with filamentous bulking [3]. 3. EFFECTS OF OZONATION ON FILAMENTS Filaments have a high surface area to volume ratio, which allows them to thrive under nutrient deficient conditions. However, the high surface area of filaments also makes them particularly susceptible to chemical attack. When ozone is applied to sludge, the lysis process occurs through the oxidation of the cell wall of the bacterial cells by ozone. It is well known that the rate of chemical reactions is enhanced by reaction systems that have large surface areas. The high surface area to volume ratios of filaments makes them especially susceptible to ozone attack. Also, even where the absolute number of filaments might be lower than that of the floc formers in a system, filaments form extensive bridging networks within and across flocs. This implies that not only single, free floating filaments are susceptible to ozone oxidation, they are also likely to be attacked by ozone molecules that penetrate the interior of the floc. The technical process of bulk control with ozone is comparable with the disintegration process. The technical integration on WWTP is similar to Figure 1. The major impact is the size of the ozone generator due to lower dosages. The ratio of RASozoned / RAStotal is in the range of 0.05 – 0.1 depending on the gas flow of the ozone generator and treatment effects. 4.1 CASE STUDY I: OZONE FOR SLUDGE REDUCTION AND FOAMING CONTROL IN ITALY The effect of sludge ozonation on filament control was tested at full scale as part of a sludge minimization project at the Lariana WWTP (25,400 m/day) located in Bulgarograsso / Italy [4]. The raw water is a mixture of industrial (mainly textile) and municipal waste water. The facility had been plagued by a significant foaming problem (see figure 2). Microbiological tests indicated that the filamentous organisms at the facility included Microthrix parvicella, Nocardiaforms, Thiotrix, Sphaerotilus and Nostocodia. The foam was stable and measured about 20 cm in height. Figure 2: Foam on aeration tank in Lariana before (left) and during (right) ozonation [4] The ozonation step was installed by Praxair (Lyso) in the RAS line and the ozone was mixed via a venturi pipe. The ozonated sludge was returned to both nitrification basins. The dosage for this project is 0.05 0.07 kgO3/kgTSreduced which is lower than former studies. The following figure 3 shows the effect of the ozonation on the abundance of different filament in the biological treatment step. Figure 3: Reduction of filaments during ozonation at Lariana WWTP [4] Nostocodia was completely eliminated in the system and did not recur throughout the test period. In the 10 week period following the commencement of the sludge ozonation tests, the average abundance levels of Microthrix parvicella, Thiotrix and Norcadiaforms were reduced by 66%, 94% and 87% respectively. The operator observed significant reduction of foaming in the aeration basin process of the wastewater treatment operation as a result of the ozonation process. The foam layer was consequently reduced. Additional effects were observed like decrease of SVI, the improvement of dewaterability of the excess sludge and no changes in the overall COD and TN removal. 5. CASE STUDY II: OZONE FOR BULKING CONTROL IN GERMANY Lyko et al [7] describes the use of ozone for bulking control at one of largest WWTP in Germany. The WWTP of Bottrop was sized for 1.300.000 PE and has an inflow at dry weather of 4.25 m3/s (mixture of municipal and industrial waste water). The problem of bulking is a seasonal problem which happens during periods with low temperature. The design includes three parallel activated sludge lanes. One of these lanes was equipped with an Ozonation step to treat approx. 6% of RAS with ozone. The RAS was pumped in the ozone plant where the ozone was mixed in by a venturi pipe. The treated RAS was returned to the denitrification basin. The most important process parameters are summarized in Table 1. Table 1: Process parameter of ozonation at WWTP Bottrop [7] Parameter Unit Value at line 3 of WWTP Bottrop Ozone in process gas wt% 8 Ozone load Kg O3/h 3 Gas flow Nm3/h 25 RAS for ozonation m3/h 300 RAS total m3/h 5,000 Operation time 1 or 2 week(s) per month The applied ozone dosage was according to Table 1 approx. 0.00167 gO3/gTSSozoned RAS. The operation of the ozone plant was discontinuously one week in operation and two weeks without operation. Figure 1 shows the changes in lane 3 and the two reference lines before and after the implementation of an ozonation step. Figure 4: SVI changing during ozonation at WWTP Bottrop [7] After the start of the ozonation the SVI level in lane 3 is on a constant level below 100 mL/g compared with the reference line where the SVI increases at the end of AlCL3-Dosing. Comparing ozone and AlCl3-Dosing the first effect on the sludge could be observed at the ozone treated lane after only two days. The effect of AlCl3-Dosing could be observed after 7-10 days. Additionally the MLSS in lane 3 could be increased to 3.5 g/L compared to 2.5 g/L in the reference lines. Since the sludge retention time of lane 3 is 17 days 14 days without ozonation are quite enough to control filamentous bacteria. The ozone generator was 1 week in operation and one week out of operation. The operating costs with ozonation are 50% less compared to the AlCl3-Dosing.