Control of activated sludge settleability using preaeration and preprecipitation

;Vat. Res. Vol. 27, No. 2, pp. 293-296, 1993 Printed in Great Britain.All rightsreserved

0043-1354/93 $5.00+ 0.00 Copyright © 1993PergamonPress Ltd

CONTROL OF ACTIVATED SLUDGE SETTLEABILITY USING PREAERATION AND PREPRECIPITATION E. Ecm~v~RL~ l, A. SECt)2 and J. FERRER3. 'EMARSA, Cno dels Anouers s/n, 46010 Pinedo, Valencia, 2Departamento de lngenieria Qulmica, Universitat de Valencia, Dr Moliner 50, 46100 Burjassot, Valencia and 3Departamento de Ingenierla Hidr~ulica y Medio Ambiente, Universidad Polittznica de Valencia, Apto. 22012 Valencia, Spain

(First received June 1991; accepted in revisedform July 1992) Abstraet--This paper deals with the addition of a preaeration and a preprecipitation stage to a conventional activated sludge process. The experiments were carried out in a pilot plant using wastewater from the city of Valencia (Spain) as feed. The effect of the F/M ratio and sludge age on sludge settleability was established. In both eases, process stability was observed to increase with respect to the conventional process. Good sludge settleabilitywas thus assured in a greater F/M ratio range. A comparative economic study was also carried out.

Key words--activated sludge settling, sludge volume index, filamentous organisms, septic sewage, bulking sludge, floating sludge, pinpoint floe, preaeration, preprecipitation

MATERIALSAND METHODS

INTRODUCTION The operation of biological treatment plants is often hampered by poor settling characteristics of the sludge. Incomplete separation in the settling stage can cause high concentrations of suspended solids in the final effluent. There are many studies in which the factors affecting activated sludge settleability and the methods proposed to control it have been investigated (Anderson, 1981; Tomlinson and Chambers, 1985). Most of them deal with laboratory prepared wastewater (Chao and Keinath, 1979; Lau et al., 1984a, b; Chudoba et al., 1985), which have led to limited application by operators and designers. In a previous paper (Echeverrla et al., 1992), the influence of the food to microorganism ratio (F/M), sludge age and sulphide concentration in the influent on sludge settleability, which was expressed as the sludge volume index (SVI), was established. The study was conducted by using domestic wastewater, in a F / M range of 0.10-0.60 g BODs/g MLVSS day, from the city of Valencia, Spain (1,000,000 inhabitants). F / M ratio values in the 0.25-0.608 BODs/g MLVSS day and 4-10 day sludge age range were found to assure good sludge settleability whenever sulphide concentrations in the influent did not exceed 1.0mg/1. This paper aims to present two possible changes in the process which lead to good settling characteristics in a wider range of operating conditions. The changes proposed involved adding an aeration or coagulation--precipitation stage prior to biological treatment. "Author to whom all correspondence should be addressed.

The pilot plant (Echeverrla et ai., 1992) consists of primary settling (effective volume 561.), mixed fiquor aeration (effective volume 791.) and secondary settling (effective volume 69.41.) with sludge recycfing. All the elements were made of acrylic plastic. In both settlers a rake operating at a rate of 0.5 rpm was employed to prevent sludge bridging. An acrylic plastic basin (effective volume 3.8 L), aerated by a ceramic diffuser, was situated before the pilot plant to study the process with a preaeration stage. A venturi meter was used to measure the air flow rate. In the case of the coagulation-precipitation alternative, chemical reactive additions took place in the influent pipe. The influent was pumped into the plant at a rate of 20 l/h and the settled sludge was returned to the reactor at the same rate. The mixed liquor volatile suspended solids (MLVSS) concentration was determined dally and excess sludge was wasted manually. The biological oxygen demand (BODs), pH and suspended solids, sulphide, phosphate, nitrate, nitrite and Kjeldahl nitrogen concentrations in the reactor influent, were measured, as well as the dissolved oxygen (DO), SVI and temperature in the reactor. Wholly isolating the effects of the different parameters was not possible, as variations in influent characteristics could not be controlled, although everything was done to prevent any superposition. The sludge volume index was the parameter used to quantify biological sludge settleability. Variations of this parameter in the 75-125 ml/g range do not entail significant alterations in the biological process. These variations do, however, give rise to low values of the correlation coe~cient when the experimental plots are fitted to the proposed curves by least squares. RESULTS

(1) Process with the Preaeration Stage The influence o f fats and sulphide concentrations in the influent on the settling characteristics of the sludge, has been highlighted by many authors (Jenkins et al., 1984; Strom and Jenkins, 1984;

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Simpson et al., 1991). Preaeration may be a reliable alternative for removal of these compounds from the influent, thus leading to more stable biological process behaviour.

Effect of the F/M ratio and sludge age The F/M ratio range studied in this series of experiments was 0.13-0.64 g BODs/g MLVSS day. The influence of the F/M ratio on sludge settleability is shown in Fig. 1, where the parabolic equation obtained on fitting by least squares is reported together with the experimental plots. Good settleability (SVI less than 125 ml/g) was found for the F/M ratio range of 0.15-0.55 g BODs/g MLVSS day. The influence of sludge age on the SVI is summarized in Fig. 2, where the SVI may be observed to present values under 100 ml/g for the 4-16 day sludge age range.

Sulphide removal A direct effect of the preaeration stage is removal of a high percentage of the sulphide present in the influent. Average removal was 50% at an air dosage of 0.64 m 3 (20°C, 1 atm)/m 3 wastewater. This fact is important for cities with extensive sewage systems which entail long detention times, and therefore septic wastewater. This septic wastewater is responsible for the predominance of filamentous microorganisms (Thiothrix, Beggiatoa spp and 021N) (Jenkins et al., 1984), which cause bulking and floating phenomena.

Fats removal Although the fats concentrations in the wastewater used in this study always lay in a range which is not considered to be problematic (32-50mg/1), the preaeration stage was observed to lead to an average fats removal of 53%, preventing these compounds from negatively affecting biological process performance.

(2) Process with the Coagulation-Precipitation Stage

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Fig. 2. Sludge volume index vs sludge age in the process with preaeration.

as a measure to increase P, N and organic loading removal. Wagger (1984) outlined the benefits of adding ferrous and aluminium salts to the aeration basin influent to prevent bulking sludge from forming. The effect of adding a coagglation-precipitation stage, prior to the biological process, on the settling characteristics of the sludge was therefore studied. Jar tests were carried out to select the chemical reactive and its operating dosage. The selection criteria involved were maximum solids removal and minimum cost. Commercial ferrous sulphate [43-45% Fe2(SO¢)3] was found to be the most suitable reactive, the dosage being 60 g/m s .

Effect of the F/M ratio and sludge age Figure 3 shows the influence of the F/M ratio on sludge settleability using this type of treatment. All the SVI values obtained for the range studied (0.21-0.80 g BODs/g MLVSS day) are under 125 ml/g, indicating good settleability. It may therefore be inferred that the biological process is not influenced by the F / M ratio in this treatment scheme. The influence of sludge age on the SVI is reported in Fig. 4, where the stability achieved by means of preprecipitation is confirmed.

Adding a coagulation-precipitation process prior to biological treatment is widely cited in the literature 8Vl ( ml/g ) 260

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Fig. I. Sludge volume index vs F/M ratio in the process with preaeration.

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Fig. 3. Sludge volume index vs F/M ratio in the process with preprecipitation.

Control of activated sludge settleability

Removal of suspended solids and BOD 5 in the primary settler

2608VI ( ml/g )

The average removal of suspended solids and BOD 5 in the primary settling tank was 63 and 48%, respectively. These findings enable the biological process to be assessed as highly stable.

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Sulphide removal in the primary settler Sulphide reacts with ferric ions to produce a very stable black ferric sulphide precipitate. Average sulphide removal was 50%. The influent sulphide concentration was in the 2.3-0.1 mg/l range. Removal of these compounds prevents bulking sludge from developing as a result of the excessive growth of filamentous microorganisms. This removal also gives rise to a gas with a low sulphide concentration in the anaerobic sludge digestion.

Phosphorus removal in the primary settler The use of a ferrous salt might lead to excessive phosphorus removal owing to ferric phosphate formarion. This removal could unbalance the nutrient requirements for suitable microorganism metabolizing. Average phosphorus removal was 54% with the dosage utilized in this study, which did not affect the nutrient requirements for the domestic wastewater used. DISCUSSION

A comparative study of the F/M ratio and sludge age influence on the SVI was carried out to establish the applicability of the alternatives (preaeration and preprecipitarion) to a conventional process. Data corresponding to the conventional process are to be found in a previous paper (Echeverria et al., 1992).

The F/M ratio The fitted curves representing the relationship between the F / M ratio and the SVI for the three cases mentioned above are shown in Fig. 5. The preaeration alternative shows SVI values below those

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Fig. 5. Comparison of the effect on the sludge volume index vs F/M ratio in the conventional process, with preprecipitation and preaeration.

obtained for the conventional process, up to F/M ratios of 0.5 g BODs/g MLVSS day. The curve found for the process with preprecipitation (nearly a horizontal straight line), shows that the F/M ratio hardly influences the SVI. The SVI values lie below 100 ml/g for the range studied. A drop in sulphide concentration in the influent is observed as a direct effect of both alternatives. Preprecipitation leads to even lower SVI values, probably owing to the supplementary coagulation effect caused by the presence of Fe 3+ in the biological treatment. This decrease in sulphide prevents proliferation of the filamentous microorganisms Thiothrix taking place. The marked decrease of SVI values for low F/M ratio values, might be justified by the theory put forward by Simpson et al. (1991), as this sulphide removal would allow trace metals to be kept at suitable levels to support unrestricted growth of floc forming microorganisms. Both alternatives may be inferred to allow operation with low F/M ratios values without the settling characteristics of the sludge being affected, while greater BOD5 removal is obtained than by the conventional process.

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Fig. 4. Sludge volume index vs sludge age in the process with preprecipitation.

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Fig. 6. Comparison of the effect on the sludge volume index vs sludge age in the conventional process, with preprecipitation and preaeration.

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Table I. Cost of treatment (pta/m3) Process ConventionalPreaeration Prepredpitation Preaerafion (EP) -0.10 -Ferric sulphate --0.66 Aeration (EP) 0.96 0.96 0.70 Dewatering 0.27 0.27 0.37 Total cost 1.23 1.33 1.73 EP: electricalpower, pta 100ffiUS $1.00.

Sludge age

The fitted curves obtained for sludge age influence on the SVI in the three cases studied, have been plotted in Fig. 6 in which the above conclusions are confirmed. The SVI minimum value range is shifted toward high sludge age values in the process with the preaeration stage, compared to those of the conventional process. SVI stabifity can be observed over the whole range studied as far as the process with preprecipitation is concerned. The stability observed in the sludge age and F / M ratio ranges studied for both alternatives, allows operation with high sludge age values (low F / M ratio). This renders the system capable of assimilating shock loads without sludge settleability being affected. Shock loading is reported as being one of the causes of bulking sludge (Barnard, 1978).

et ai.

High coagulant dosage can lead to excessive removal o f organic load and phosphorus, disturbing the biological process and producing pin-point flue and bulking phenomena, respectively. Low increase in costs justifies the generalized use of a preaeration stage. Using a preprecipitation stage is only justified in wastewater treatment plants with disfunctions in the biological process as a result of alterations in influent characteristics or deficiencies in the facilities. ~NCES

Anderson J. A. (1981) Thickening performance of activated-sludge settlement tanks. Wat. Pollut. Control 4, 521-528. Barnard J. L. (1978) Solving sludge bulking problems. War. Pollut. Control 4, 103-106. Chao A. C. and Keinath C. M. (1979) Influence of process loading intensity on sludge clarification and thickening characteristics. Wat. Res. 13, 1213-1223. Chudoba J., Cech J. S., Farkac J. and Grau P. (1985) Control of activated sludge filamentous bulking. Experimental verification of a kinetic theory. Wat. Res. 19, 191-196. Echevcrria E., Seco A. and Ferret J. (1992) Study of the factors affecting activated sludge settling in domestic wastewater tr©atment plants. Wat. Sci. Technol. 25, 273-279. Jenkins D., Richard M. G. and Neethling J. B. (1984) Causes and control of activated sludge bulking. Wat. Pollut. Control 56, 455-472. Lau A. O., Strom P. F. and Jenkins D. (1984a) Growth COMPARATIVEECONOMIC STUDY kinetics of Sphaerotilus natans and a floc former in pure and dual continuous culture. War. Poilut. Control 56, The increase in costs owing to the introduction o f 41-51. preaeration and preprecipitation stages was assessed. Lan A. O., Strom P. F. and Jenkins D. (1984b) The This assessment is summarized in Table 1. competitive growth of floe-forming and filamentous bacUsing a preaeration stage involves a rise of about teria: a model for activated sludge bulking. Wat. Poilut. Control 56, 52-61. 1% in overall operating costs, which are estimated at 10 pta/m 3. In the case of the preprecipitation stage Simpson J. R., List E. and Dunbar J. M. (1991) Bulking sludge: a theory and successful case histories. J.I.W.E.M. costs increase by about 5%. 5, 302-311. Strom P. F. and Jenkins D. (1984) Identification and significance of filamentous microorganisms in activated CONCLUSIONS sludge. War. Pollut. Control ~ 449-459. Adding a preaeration or preprecipitation stage Tomlinson E. J. and Chambers B. (1984) Control strategies for bulking sludge. Wat. Sci. Technol. 16, 15-34. modifies the influence of the F / M ratio and sludge Wagner F. (1984) Studies on the causes and prevention age on the SVI increasing the operating range and of bulking sludge in Germany. War. Sci. Technol. 16, stability of the biological process. 1-14.