Iwa international Specialist Conference

Strategies to Maximise Pathogen Kill

Roger Matthews, Thames Water Utilities, Gainsborough House, Manor Farm Road, Reading, Berkshire, England, RG2 0JN

Manocher Asaadi, Thames Water Utilities, Spencer House, Manor Farm Road, Reading, Berkshire, England, RG2 0JN
[e-mail], [e-mail]

Key Words: Anaerobic Digestion, Pathogen Kill, Operation, Secondary

The new guidelines for sludge recycling to land in Britain will be implemented from early 2003. The Regulations define two classes of biosolids, ‘treated sludge’ and ‘enhanced treated’ sludge (see table 1).

TABLE 1

	Log Reduction of Indicator Organism (E-coli)	Maximum Allowable Concentration (MAC)
Treated Sludge	2 log	10^5 per gDS
Enhanced Treated Sludge	6 log	10^3 per gDS

In the UK, mesophilic anaerobic digestion is predominantly used for treatment of sludge. Studies by some of the UK Water Companies have established that a 1.5 log kill can be achieved in the digestion process but a further period of secondary storage is required to achieve the necessary 2 log kill and the MAC standard required by the Regulations.

In Thames Water, twenty-three digestion sites have been studied to investigate the possible relationship between their pathogen kill against various operating parameters including retention time, temperature organic loading rate, secondary storage time and mechanical dewatering. Bench scale study has also been carried out to investigate the impact of pre-treatment processes such as acid phase digestion on pathogen reduction.
In this paper three approaches adapted in TW to improve pathogen reduction of anaerobic digestion process have been described.
At one site with a population equivalent of 1.7 million refurbishment programme enabled eight of the digesters to be operated at eight different operating regimes, with temperatures up to 38^oC and retention times from 12 to 19 days with the same feedstock. This site has no secondary storage, and to construct storage for a site that produces 3,000 m3 of digested sludge per day, would be prohibitively expensive. Therefore an exercise to maximise the pathogen kill was begun to see if better than 1.5 log kill experienced by other Water Companies could be improved upon. The data is collected showed that it was not achievable and an alternative strategy was required.
In the UK, anaerobic digestion operated in two stages, primary digestion followed by secondary batch storage. Primary digestion stabilizes sludge by destroying the organic solids and producing biogas whilst during the secondary storage consolidation and pathogen reduction occur. An alternative approach was adapted where secondary batch storage tanks were converted to run on continuous mode of operation. This resulted in reduction of around 20% volume required whilst maintaining good pathogen reduction performance.

Another approach was to use a pre-treatment process in front of conventional anaerobic digestion. Bench scale trials showed that using short retention time MAD, known as acid phases digestion, operating at high organic loading resulted in high VFA concentration. This proved to cause effective E.coli kill. The combined process of APD and MAD produced a ‘treated’ product without the need for secondary storage. Based on the result of this study a full-scale APD has been built and is undergoing commissioning at a 300,000 STW.

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38.

Improvement of sewage sludge quality :

Combination of ozone and anaerobic digestion processes for the treatment of Polycyclic Aromatic Hydrocarbons (PAHs) present in sludges.
Bernal-Martínez, A., Carrère*, H., Patureau, D. and Delgenès, J.P.

Institut National de la Recherche Agronomique (INRA), Laboratoire de Biotechnologie de l'Environnement, Avenue des Etangs, F- 11100 Narbonne, France

*Corresponding author
New wastewater treatment regulations imply an important sewage sludge production increase. Their treatment and disposal may constitute a great problem for the environmental protection and local communities management. A great number of community carry out sludge land application which represents 60 % of sludge disposal in France. However, its use can present some potential risks owing to the accumulation in agricultural soil of trace metal or organic micro-pollutants. Among organic micro-pollutants, polycyclic aromatic hydrocarbons (PAHs) are targeted by an european regulation which sets their maximum concentration for land application. PAHs are a group of bioaccumulator organic compounds, toxic and persistent, that contain two or more merged aromatic rings. They are widely distributed in the environment due to numerous sources of pollution. They are known to be biorecalcitrant compounds in reason of a low bioavility. Indeed, as they present a low solubility in water and a high affinity for organic compounds, they are concentrated into sewage sludge during wastewater treatment. A previous work in our laboratory investigated their removal during the sludge stabilisation by anaerobic digestion (Trably et al., 2002). About 50% reduction of 13 PAHs was achieved. Moreover, ozonation has been applied to eliminate some PAHs in aqueous solutions (Luster et al., 2002, Caderera et al., 2001, Trapido et al.,1995). This work deals with the combination of anaerobic digestion and ozonation to remove PAHs naturally present in sewage sludges. 13 PAHs were considered : Fluorene, Phenanthene, Anthracene, Fluoranthene, Pyrene, Benzo(a)Anthracene, Chrysene, Benzo(b)Fluoranthene, Benzo(k)Fluoranthene, Benzo(a)pyrene, Dibenzo(a,h)Anthracene, Benzo(g,h,i)Perylene, Indeno(1,2,3,cd)Pyrene). Another valuable result of this study is the reduction of the sludge amount compared to the one obtained during anaerobic digestion as already shown by Battimelli et al., 2002 and Goel et al., 2002.

In a first time, ozonation was used as a post-treatment of anaerobic digestion to investigate the enhancement of PAHs removal by ozone treatment. In a second time, according to the assumption that ozonation can increase the PAHs biodegradability, some ozonated sludge will be recycled into the anaerobic digester. The objectives of this study were: (1) to determine the optimum post-treatment O₃ dose for the removal of 13 PAHs, (2) to examine the effect of pH, (3) to combine O₃ with H₂O_2, with an organic solvent (methanol) and with 3 surfactants (brij, tergitol and tyloxapol) with the aim of increasing the PAHs solubility in aqueous phases and (4) to evaluate the feasibility of combination of the anaerobic digestion with the ozonation to eliminate PAHs.

All ozonation experiments were carried out with a mixture of primary and secondary sludges polluted in situ with PAHs which were digested in an 20 L completely mixed continuous anaerobic reactor. The hydraulic retention time was 40 days and the organic loading was 1 kg COD.m^-3.d^-1. PAHs concentrations of the effluent samples from the anaerobic digestion and after the ozonation were determined according a method previously tested and validated in the laboratory by the use of certified matrice (Trably et al., 2002).

For each experiment, 450 mL of digested sludge was ozonated in a 2 L bubble batch reactor. The ozone was generated from pure oxygen by an Ozat CFSI generator and injected into the bottom of the reactor through a thin bubble diffuser. The ozone concentrations in gas phase (before and after the reaction with sludge) was measured each 30 seconds during the oxidation (with an UV BMT 963 analyser) in order to determine the ozone consumption. The gas flow rate was 1 L/min, O₃ concentration was from 50 to 60 mg/L, and the duration varied from 15 to 45 min according to the applied ozone dose. The part of PAHs lost by stripping during ozonation was determined by adsorption on an ORBO cartridge installed on the gas outlet.

Figure 1 shows the effect of ozone dose on PAHs removal from digested sludge. The selected dose for further works was 1.5 gO₃/L (55.4 % of PAHs removal) as higher doses led to higher stripping and to moderate HPA removal. pH had no influence in the removal of these compounds during the ozonation. Different concentrations of H₂O₂, methanol and surfactants (brij, tergitol and tyloxapol) were added to the sludge in order to enhance the PAHs removal. Results are shown in table 1.


Figure 1: Effect of ozone dose on PAHs removal

	Before the anaerobic digestion	After the anaerobic digestion	1.5 gO3/L	O3+H2O2	O3+ MeOH	O3+ brij	O3+ tergitol	O3+ tyloxapol
µg/L PAHs	351.95	205.55	156.96	74.46	159.57	90.66	83.11	80.93
% removal		41.6	55.40	79	54.65	74.24	76.38	77
g/L TSS	23.35	12.95	11.38	11.17	9.72	10	10.5	9.4

Table 1. PAHs removal with 1.5 gO₃/L and addition of H₂O₂, methanol and surfactants (sum of the 13 PAHs concentrations, TSS: total suspended solids)
The addition of a highly oxidant agent, such as H₂O_2, increased the removal up to 79%. Comparable performances were obtained by the use of surfactants which improved PAHs solubility in the aqueous phase. Addition of methanol did not led to an increase of the PAHs removal probably because ozone can easily reacts with alcohols.

Finally, in order to evaluate the feasibility of the combination of the anaerobic digestion with ozonation, the biodegradability and biotoxicity of the sludges ozonated in presence of surfactants and H₂O₂ was measured.

References

Battimelli A., Millet C., Delgenès J.P and Moletta. (2002). Anaerobic digestion of waste activated sludge combined with ozone post-treatment and recycling. 3^rd International Symposium Anaerobic Digestion of Solid Wastes. Munich/Garching Germany. September 18-20.

Calderara V., Jekel M. and Zaror C. (2001). Kinetics of ozone reactions with 1-naphthalene 1.5 naphthalene and 3-nitrobenzene sulphonic acids in aqueous solutions. Wat. Sci. Techn. 44(5):7-13.

Goel-Rajeev., Yasui H.,. Noike T. (2002). Closed Loop Anaerobic Digestion Using Pre/Post Sludge ozonation and Effect of Low Temperature on Process Performances. 3^rd International Symposium Anaerobic Digestion of Solid Wastes. Munich/Garching Germany. September 18-20.

Luster-treasley S. L., Yao J.J., Herner H.H., Trosko J.E and Masten S.J. (2002). Ozonation of Chrysene: Evaluation of Byproduct Mixtures and Identification of Toxic Constituent. Environ. Sci. Technol. 36. 869-876.

Trably E., Patureau D. and Delgenès J.P. (2002). Optimization and validation of a high-reproducible method for Polycyclic Aromatic Hydrocarbons analysis in sewage sludges samples. Submitted in Int. Env. Anal. Chem.

Trapido M., Veressinina Y., Munter R. (1995). Ozonation and advanced oxidation processes of polycylic aromatic hydrocarbons in aqueous solutions-A Kinetic study. Environmental Technology. 16:729-740.

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BIOTECHNOLOGY OF INTENSIVE AEROBIC BIOCONERSION OF SEWAGE SLUDGE AND FOOD WASTE INTO FERTILIZER