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Chlorination of Recycling...

Introduction...

The filaments protruding from the flocs into the bulk liquid can be selectively damaged by strong oxidants. The filaments are exposed to lethal concentrations of chemicals while the concentration inside flocs is sublethal thanks to the transport limitation and consumption of the toxicants during the transport through the floc. In practice, chlorine and hydrogen peroxide have been used successfully. The effective chlorine dosages are in the range of 1 - 10 g Cl₂ / kg MLSS . day. The use of chlorine for this purpose is widespread in the USA, while in Europe the method is applied only occasionally because of the fear that addition of chlorine involves the risk of uncontrollable formation of chlorinated organic compounds which are discharged into the receiving waters. In South Africa, ozonation of activated sludge was studied for bulking control, with excellent results. The filaments can be also damaged or their length decreased mechanically by a shear stress. Under such conditions the growth of filamentous microorganisms is hindered. Various pieces of mechanical equipment such as blenders, high speed pumps or aerators can be used for that purpose. Surprisingly, a kind of shear stress can also be evoked by a continuous contact of filaments with small inert particles added to the mixed liquor, which are kept in suspension together with the flocs. A sawdust was successfully used for the suppression of filamentous bulking even in completely mixed tanks.

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Damaging the Filamentous Microorganisms...

A group of nonspecific methods is especially aimed at those filamentous microorganisms which preferentially grow outside the flocs. The filaments protruding from the flocs can be selectively damaged by strong oxidants. The filaments are exposed to lethal concentrations of chemicals in the bulk liquid while the concentration inside the flocs is sublethal because of the transport limitation (diffusional resistance) and consumption of the toxicants during transport through the floc.

The most traditional oxidant for these purposes (even before World War II is chlorine. The use of chlorine is especially advocated by Jenkins and his coworkers in the US. Bulking control with chlorination was also intensively studied in South Africa, mainly with regard to the impact on nutrient removal and organohalogen formation. In Europe chlorination is used more in the United Kingdom than on the Continent. In Germany and Central Europe, chlorination is recommended only as an emergency measure. The main reason is the fear of organohalogens discharged in final effluents to receiving waters which are already sufficiently loaded by AOX from industry.

As chlorine is a very dangerous chemical, both for activated sludge microorganisms and for the environment, chlorination should follow some rules, and can be done only by trained staff. Jeakins et al. recommend the titration of SVIs. The target value of SVI (SSVI, DSVI) under which the plant can be successfully operated should be established before chlorination. Chlorine is dosed only when the target SVI is significantly and consistently exceeded. The following parameters were found to influence the chlorination efficiency ;

(1) Overall mass dose rate of chlorine (g Cl₂ / kg MLSS . day)
r_{X - Cl2} = Mass of chlorine dosed per day / Mass of activated sludge in system

The values of r_{X - Cl2} found in the literature range from 1 - 15 g Cl₂ / kg MLSS . day. However, dose rates above 5 are rather high, and sludge quality during the chlorination should be carefully checked (see below). Chlorination should start with lower dose rates especially when it is done for the first time in a particular plant.

(2) Chlorine concentration at the dose point (mg / L)
S_Cl2 = Mass of chlorine dosed per day / Flow-rate past dose point

The local chlorine concentration should not exceed 15 - 20 mg / L, otherwise the internal parts of flocs can be damaged. The actual values of chlorine concentration will thus depend on the activated sludge concentration X. This is expressed by means of the next parameter.

(3) Local specific mass dose at the dose point (g Cl₂ / kg MLSS)
D_{X - Cl2} = S_Cl2 / X

The value of local specific mass dose is affected also by the frequency of exposure ;

(4) Frequency of exposure of activated sludge to chlorine dose (day ^{- 1})
F = r_{X - Cl2} / D_{X - Cl2}

The damage of filaments by chlorine can only be effective when the filaments are exposed to chlorine several times a day. The recommended F value is more than 2.5 - 3.0 day ^{- 1}.

Because of the great aggressiveness and reactiveness of chlorine, this chemical should only be added to a point in the activated sludge system where it can be specifically targeted on filamentous microorganisms. The most suitable dosing point seems to be the return activated sludge stream, especially in places with high turbulence. The chlorine dose should be reduced as quickly as possible. If chlorine survives in the RAS stream for a period of time, there is a danger that this reactive chemical will not only penetrate into deeper layers of activated sludge floc, but that it will also enter the main aeration basin with the RAS. This will result in substantial losses of chlorine, which will react with organic compounds from wastewater instead of with the filaments. In addition, the chlorination of wastewater organics generate more organohalogens than can be released from the chlorination of filamentous biomass.

The progressive effect of chlorine on the activated sludge biocenosis should always be carefully and regularly checked by means of microscopic observation. Microscopic examination can, in most cases, indicate chlorine overdosing well before plant performance deteriorates. According to Jenkins, there are some early warning signs of chlorine overdosing.

• Intracellular granules of storage products like PHB and sulphur have completely disappeared because they were oxidized by chlorine
• Empty sheaths, or empty places within the ensheathed filaments can be observed
• There is total absence of fllamentous microorganisms protruding from the focs, there are small (broken down) flocs, and no higher microfauna are present (this microscopic picture corresponds to turbid supernatant after settling)

When this last sign is observed, chlorine dosing should be stopped immediately until the microflocs reaggregate back to structures capable of settling.

The susceptibility of individual filamentous microorganisms to the effect of chlorine is also frequently discussed. Howevcr, as far as the filaments growing mostly outside the activated sludge floc, are concerned, there is no reason for any significantly different response. The effect of chlorine is based on chemical reaction of this oxidizing agent with organic compounds in the filament biomass. As the chemical composition of the biomass of different filamentous microorganisms should be similar, the effect of chlorination should also be similar. It can be concluded that chlorine generally affects all Eikelboom types of filaments in a similar manner. Nevertheless, there are some indications that Microthrix parvicella could be more tolerant to toxic chlorine doses. Some researchers found that a chlorine dose rate of 4 g / kg . day had very little effect on Microthrix parvicella, and at a rate of 8 g / kg . day it took 15 days before Microthrix parvicella showed significant decay. The chlorination did not result in a complete disappearance of this filamentous microorganism. It is interesting to note that Type 0092, another representative of all zones growers, also exhibited increased resistance to chlorine. On the other hand, some authors found that chlorine can control the growth of Microthrix parvicella at standard specific mass doses.

If bulking control with chlorination is applied to a nutrient removal activated sludge system, a temporary deterioration of nitrification and EBPR efficiency has to be expected. Some researchers used chlorine at the Fishwater Flats Water Reclamation Works in Port Elizabeth, South Africa for controlling bulking caused by Types 0041 and 0092. Due to certain limitations at the plant, the average dose rate r_{X - Cl2} was only 3.7 g / kg . day and the frequency of exposure F = 1.6. Nevertheless, even at those low figures activated sludge settling properties improved after 2 weeks of chlorination, but nitrification was adversely affected. During the chlorinati6n period, the effluent ammonia concentration increased from about 1 mg / L to more than 20 mg / L. Once chlorination ceased, nitrification recovered within a few days. As the chlorination did not solve the bulking problem and had to be repeated at the plant, the authors could also repeatedly observe the similar course of nitrification deterioration and recovery.

Some researchers confirmed in a laboratory study that nitrifiers are more susceptible to inhibition from chlorine than filamentous microorganisms (unfortunately, not specified to types). While the minimum dose rate r_{X - Cl2} which affected the growth of filaments was 2.5 g / kg . day for 2 days, nitrification was adversely affected at the dose rate of 1.5 g / kg . day. The authors recommend pilot tests before full-scale chlorine application to find the threshold dose rate with respect to nitrification.

Some researchers studied the effect of chlorine on polyphosphate accumulating (poly-P) bacteria in a modified UCT biological nutrient removal activated sludge process. The chlorination was targeted on Types 0092 and 0914 and Microthrix parvicella as filaments causing the bulking problems in the lab-scale modified UCT plant. The authors observed a precipitous decline of EBPR mechanism after 16 days of chlorination at the chlorine dose rate of 8 g / kg . day. However, phosphorus removal recovered to its normal values within 5 days after cessation of chlorination. Since, in this particular case, chlorine was added to the mixed liquor entering the oxic reactor of the UCT system, only the phosphorus uptake mechanism was affected by chlorination.

Possible organohalogen formation during sludge bulking control with chlorine is the most frequent argument against chlorination. However, quantitative data supporting this argument are rather scarce in the literature. The most detailed study of this problem was done by van Leeuwen and van Rossum in South Africa. They found that the concentration and the forms of trihalogenmethanes formed in chlorination depends on the components oxidized by chlorine. The lowest specific production of chloroform, CHCl₃, was observed for the chlorination of activated sludge biomass only (concentrated, washed and resuspended activated sludge). On the other hand, the highest specific trihalogenmethane formation was observed when a filtered activated sludge system effluent was chlorinated. In this case, other trihalogenmethanes like CHBrCl₂ and CHBr₂Cl were also formed. The organohalogens are thus more readily formed from substances in the effluent (dissolved and colloidal unbiodegradable compounds either originating from treated wastewater or produced by activated sludge microorganisms) than from the reaction of chlorine with biomass. The authors also found that the concentration of organohalogens in effluents from the activated sludge process can be significantly lowered because of air-stripping of these volatile compounds in aeration basins. However, from the point of view of environmental contamination, the final effect will be the same.

Nevertheless, one important conclusion can be drawn from the studies by van Leeuwen and van Rossum, namely, that chlorine is added to the concentrated return activated sludge and reacts immediately with the biomass, the concentration of organohalogens in the final effluent can be maintained below the standards. Short-term chlorination during the bulking period at an activated sludge plant certainly will not be more harmful to the environment than poor effluent from the plant caused by bulking.

There were some attempts to replace chlorine with other oxidizing agents for bulking control. The use of hydrogen peroxide, H₂0₂, is recommended in the manual by Jenkins et al. However, this author tried to cure filamentous bulking in his pilots several times, and always with little success. Hydrogen peroxide does not seem to be selective for filaments protruding from the flocs. Because it is less reactive than chlorine, hydrogen peroxide may penetrate deeper in the flocs and also affect the biomass of floc-formers. When hydrogen peroxide was used in our models, a destruction of flocs was often observed at H₂0₂ dose rates effective for filamentous microorganism control.

In South Africa, van Leeuwen and Pretorius tested ozone for sludge bulking control. Ozone is a more powerful oxidizing agent than chlorine or hydrogen peroxide, and no toxic substances are formed during ozonation. The speciflc ozone mass dose effective for hulking control in a PHOREDOX-type nutrient removal activated sludge system was 4 g O₃ / kg MLSS. Ozone was dosed to the oxic zone of the PHOREDOX-type plant. The results indicate that ozonation can promote nitrification and removal of refractory organics. The EBPR mechanism is not affected by 0₃.

Long trichomes are not necessarily the only growth form for filamentous microorganisms. In practice, there are two big questions about the control of filamentous bulking : Can we by some action affect the already developed trichomes ? and Can we even prevent the microorganisms from growing in long trichomes ?

Long-term experiments carried out at the Prague Institute of Chemical Technology have indicated that there is a simple way to prevent microbes from forming long filaments. A kind of shear stress evoked by the continuous contact of filaments with small inert particles added to the mixed liquor and kept in suspension together with the flocs proved to be effective in controlling the development of long trichomes. For this purpose, sawdust was successfully used for the suppression of filamentous bulking, even in completely mixed tanks. Numerous samples of activated sludges originating from both municipal and industrial wastewater treatment plants have been tested with this method. A volumetric addition of the 2 - 5 % sawdust to the mixed liquor always resulted in a very quick (less than two weeks) disappearance of nuisance filamentous microorganisms regardless of their type. Until now, we have not found a single type of filamentous microorganism which is resistant to this mechanical shear stress resulting from contacts of trichomes with sawdust particles. Mechanical damage of trichomes was observed both for sheathed and unsheathed filamentous microorganisms. When the sawdust was added to activated sludges with a well-developed population of filamentous microorganisms, a temporary increased turbidity of the final effluent seemed to be the only negative sign observed within the first days. The increased turbidity was apparently caused by small fragments of previous long trichomes.

A positive effect of increased turbulence in pipelines is well-known to practitioners. Various pieces of mechanical equipment such as blenders, high speed pumps and aerators have a destructive effect on filaments. For instance, Martin et al. demonstrated the positive effect of a return activated sludge passage through a high-turbulence Venturi tube. Filamentous growth was suppressed. However, the mechanical shear stresses should be limlted so as to avoid the production of unsettleable microflocs.