WaterSubject

Most of the paper mills in the UK use waste paper or imported pulp as their raw materials with only a small amount of wood pulping being undertaken. The production of paper and board generates a significant volume of wastewater. Depending on the nature of the product, this can be as high as 80 m³/dry tonne. It can also be highly polluting as seen. The majority of paper mills in the UK use activated sludge plants to treat their wastewater and a recent survey showed that these plants were very prone to bulking problems.

Although the activated sludge process has been used for wastewater treatment for almost a century, the phenomenon of sludge bulking is still not properly understood. This means that controlling the settlement characteristics of activated sludge, something which is vital to the correct operation of the overall process, is an imprecise art. It is generally considered that activated sludges will bulk when there is a dominance of the sludge ecology by filamentous bacteria. Filaments are an essential element in the activated sludge floc structure in that they act as a ‘backbone’ for the floc. It is when they grow out from the floc that problems occur. Specifically, when the total filament length exceeds around 60000000 ?m/mg total suspended solids (TSS) the sludge settlement properties deteriorate rapidly. A 2-year survey of sludges from activated sludge plants treating wastewaters from paper mills in the UK showed that the most common filaments associated with bulking were Type 021N (now classified as Thiothrix eikelboomii), Thiothrix and Nostocoida limicola ll.

However, reported that the presence of filamentous bacteria was not always associated with poor settleability and recent work has demonstrated that surface characteristics, such as charge and hydrophobicity, can have a significant effect of the bioflocculation of activated sludge. Work also demonstrated that both surface charge and hydrophobicity were affected by the solids’ retention time with surfaces being less negatively charged and having a higher hydrophobicity at longer, >15 days, retention times.

The manipulation of the surface hydrophobicity of activated sludges has been suggested as a technique for controlling another microbiological problem in activated sludge plants, the formation of stable foam. Tested a range of chemicals and found that, from the initial screening, the most effective were talc, bentonite and zeolite. However, the use of bentonite did not alter the cell surface hydrophobicity. The effect of talc and zeolite on the cell surface hydrophobicity was not tested. An amended talc has also been used as an agent for controlling sludge bulking although the rationale behind its use was to create a floc with a greater density.

The natural talc-product Aquatal^® has been used since 1996 in numerous applications in plants as a recognised and effective means of solving settling problems. In Germany, talc has been proposed in reports of the ATV (the German Wastewater Association) and in the literature as an unspecific means to treat bulking problems. Results of full-scale studies in communal and industrial wastewater treatment as well as in paper mill wastewater treatment have been published. Some important full-scale experiences in the paper industry are listed.

This paper reports the results of a laboratory study and of full-scale investigations into the control of the biomass in activated sludge plants using talc. The lab-scale unit was fed with simulated paper mill wastewaters and the surface characteristics of activated sludge were examined, in particular, how the hydrophobicity of the sludge surfaces varied and how this affected the settlement characteristics. The full-scale investigation monitored the effect of talc on the settlement characteristics of the sludge and the treatment efficiency in a large full-scale wastewater treatment plant treating wastewater from pulp production.

Laboratory-scale plant

One laboratory-scale activated sludge plant was used. The aeration tank had a working volume of 10 L (50 cm × 20 cm × 10 cm) and the settlement tanks one of 4.0 L. Aeration was provided at four points along the tank with fine bubble diffusers and the air flow was controlled by a rotameter (C.T. Platon Ltd., Basingstoke, UK, 0–1.2 L/min). In addition the tank was stirred (RW16, IKA Labortechnik, Janke and Kunkel GmbH, Germany). The settlement tank had conical bottoms (12.5° floor slope) and was stirred at 0.0002 g using a PREMOTEC motor (Mclennan Servo Suppliers Ltd., Camberley, UK). The addition of feed and the recycling of the settled sludge were achieved with peristaltic pumps (Model 313, Watson Marlow Ltd., Falmouth, UK). The plant had been operating with the simulated paper mill wastewater for several months prior to this study and was, therefore, fully acclimatised. The mixed liquor suspended solids’ concentration was maintained at around 3500 mg/L by wasting sludge once a week. The sludge age did vary but, effectively, it was 20 days. The hydraulic retention time was maintained at 20 h.

Substrate

One basic liquor was generated by pulping (Weverk, Wennbers, Karlstad, Sweden) blends of waste paper as recommended by the paper company participating in the research programme (location: Flint, UK). The liquor, which was fed to the reactor, was obtained by pulping a blend of cardboard (90%) and newsprint (10%). Sodium acetate (1.5 g/L) was added to the liquor to increase the chemical oxygen demand (COD) to about 1500 mg/L, which is a value comparable with that found at paper mills. In this case, 15.64 g of raw fibre were used for every litre of feed being prepared. Nutromex (OMEX Environmental Ltd., King’s Lynn, UK), a mixture of urea and the di-ammonium hydrogen phosphate, was added to maintain a C : N : P ratio of 200 : 5 : 1.

Filament ecology

Light microscopic examinations were made with an Olympus Vanox microscope (Melville, NY, USA) and the identification of the filamentous bacteria was based on the staining/morphological method described. The filament abundance (FA) was also measured using the arbitrary scale recommended. Thus, a sludge with little or no filament outgrowth was scored as 1 while a sludge heavily dominated by filamentous species was given a score of 5. Scanning electron microscopy was carried out at the Centre for Electron Microscopy at the University of Birmingham.