Excess Sludge Production Research Articles

Operating strategies for aerobic fluidized bed reactors

This research demonstrated that an aerobic fluidized bed reactor can be operated either to maximize substrate removal, to minimize solids production or to treat intermittently produced wastewater. The operational mode is controlled by the quantity of biomass attached to the media. A thinner biomass encourages endogenous respiration and minimizes excess sludge production. Increasing biofilm thickness increases substrate removal efficiencies up to the point at which the diffusion of substrate, nutrients or oxygen becomes limiting. A thick biofilm was found to remain active even after an extended non-loading period, most likely because of endogenous respiration. Upon the introduction of substrate after the non-loading period, the substrate was immediately removed. This indicates the potential of using a fluidized bed reactor for applications that produce only intermittent wastewater flows, such as those found in many industries and hazardous waste remediation systems.

Phosphate Removal from Municipal Wastewater with Low Lime Dosage

The potential use of lime in the pre-treatment of municipal wastewater before the biological process was investigated. The aim of this work was to develop an equilibrium model for predicting optimal treatment conditions to assure satisfactory P removal efficiency and to minimize carbonate precipitation, that causes high lime dosage and excessive sludge production. Model calibration was performed with the results of lime addition tests to actual sewage. The proposed equilibrium model for lime addition to municipal wastewater assumes phosphate and carbonate solubility controlled by metastable Ca4H(PO4)3 and amorphous CaCO3, respectively. Model predictions suggest that lime addition up to pH 9 may assure efficient removal of phosphorus and prevent carbonate precipitation, even in wastewater with high alkalinity content.

An activated sludge process without excess sludge production

An activated sludge process which produces no excess sludge was developed. The process is very simple as a small amount of return sludge is ozonated and then returned to the aeration tank. The ozonation enhances biodegradability of activated sludge, which is biologically oxidized in the aeration tank.A full-scale plant for treating 450m3/d of municipal wastewater was constructed and has been operated successfully for 9 months. The amount of excess sludge eliminated is directly proportional to the amount of ozone dosed to the sludge. At the ozone dosing rate of 0.034 kg/kg-SS, complete elimination of excess sludge has been achieved when 4 times more amount of sludge is ozonated than that of the excess sludge expected in the treatment without ozonation. After 5 months of operation without any withdrawal of excess sludge, small amount of inorganic substances like sand and silt accumulated in the sludge. On the other hand, inert organic substances does not seem to accumulate. As for effluent quality, BOD and nitrogen were kept good. Although effluent SS was 2-15 mg/l higher compared to a control without ozonation, it has been well below the discharge limit.

A full-scale operation of a novel activated sludge process without excess sludge production

The authors have presented a new concept of excess sludge elimination treatment with recirculation of sludge via ozonation in the activated sludge process. This paper is intended to clarify the potential application of the process to municipal and industrial wastewater treatments. In a full-scale operational experiment lasting 10 months under 550 kg/d of BOD loading, no excess sludge was needed to be withdrawn and no significant accumulation of inorganic solids occurred in the aeration tank. Most of the inorganic compounds in the sludge were released to the soluble phase. Material balance indicated that one-third of ozonated sludge was mineralized via the recirculation treatment, and thereby the requirement of sludge mass to be treated was 3.3 times as much as sludge to be eliminated. Effluent TOC was slightly higher than under the conventional activated sludge process, indicating that refractory TOC was released from the sludge eliminated by treatment. The amount of released TOC corresponded to less than 2 weight % of eliminated sludge under recirculation rates below 30% of total biomass in the aeration tank in a day, but increased at higher recirculation rates. The operation costs associated with the process were estimated to be lower than those of conventional dewatering and disposal.

Sustainable sewage treatment plants - application of nanofiltration and ultrafiltration to a pressurized bioreactor

Based on earlier studies and experiences the Institute of Sanitary Engineering, Water Quality and Waste Management at the University of Stuttgart has developed a new technology to purify wastewater to a quality suitable for reuse as industrial water. This technology consists of a pressurized bioreactor under 3 bar with an ultrafiltration unit for sludge retention coupled with a nanofiltration unit. The concentrate retained in the nanofiltration is fed back to the bioreactor resulting in an enhanced biodegradability and a better quality of the effluent. The flux through the ultrafiltration membrane ranged from 100 to 200 l/(m3h); that through the nanofiltration membrane was around 0.05 l/(m3h). While attaining a very high biomass concentration (20 to 40 kg MLVSS/m3) in the bioreactor the excess sludge production is remarkably small with 0.7 to 1.0% of the influent COD.

Domestic sewage treatment in an upflow anaerobic sludge blanket - sequencing batch reactor system

Domestic sewage treatment in up-flow anaerobic sludge blanket (UASB) reactors has presented significant results in tropical regions. This reactor configuration has proven to be efficient in removing organic matter and total suspended solids as well as in producing smaller amounts of excess sludge than conventional aerobic reactor. On the other hand, its effluent often requires to be post-treated in order to achieve the emission standards. This paper is concerned with the alternative proposed here for treating domestic sewage in a combined anaerobic-aerobic system composed of an UASB reactor followed by sequencing batch aerobic reactors (SBR). In such a system, the UASB reactor removes considerable fraction of the influent organic matter, while the SBRs oxidize part of the remaining organic matter and ammonium nitrogen. A proper system operation would also permit the removal of nutrients (N and P). In order to investigate on the performance of this system for sewage treatment, a bench scale installation fed with synthetic substrate simulating domestic sewage was operated continuously during 38 weeks. The results permit to confirm the hypothesis proposed, since the system has consistently produced high quality effluents (BOD5 and VSS lower than 10 mg.L-1). The results also indicate that such combined anaerobic-aerobic system compete favorably with conventional aerobic systems in three essential cost features: energy consumption, excess sludge production and nutrient removal.

Domestic sewage treatment in an upflow anaerobic sludge blanket - sequencing batch reactor system

Domestic sewage treatment in up-flow anaerobic sludge blanket (UASB) reactors has presented significant results in tropical regions. This reactor configuration has proven to be efficient in removing organic matter and total suspended solids as well as in producing smaller amounts of excess sludge than conventional aerobic reactor. On the other hand, its effluent often requires to be post-treated in order to achieve the emission standards.This paper is concerned with the alternative proposed here for treating domestic sewage in a combined anaerobic-aerobic system composed of an UASB reactor followed by sequencing batch aerobic reactors (SBR). In such a system, the UASB reactor removes considerable fraction of the influent organic matter, while the SBRs oxidize part of the remaining organic matter and ammonium nitrogen. A proper system operation would also permit the removal of nutrients (N and P).In order to investigate on the performance of this system for sewage treatment, a bench scale installation fed with synthetic substrate simulating domestic sewage was operated continuously during 38 weeks. The results permit to confirm the hypothesis proposed, since the system has consistently produced high quality effluents (BOD5 and VSS lower than 10 mg.L-1).The results also indicate that such combined anaerobic-aerobic system compete favorably with conventional aerobic systems in three essential cost features: energy consumption, excess sludge production and nutrient removal.

A full-scale operation of a novel activated sludge process without excess sludge production

The authors have presented a new concept of excess sludge elimination treatment with recirculation of sludge via ozonation in the activated sludge process. This paper is intended to clarify the potential application of the process to municipal and industrial wastewater treatments. In a full-scale operational experiment lasting 10 months under 550 kg/d of BOD loading, no excess sludge was needed to be withdrawn and no significant accumulation of inorganic solids occurred in the aeration tank. Most of the inorganic compounds in the sludge were released to the soluble phase. Material balance indicated that one-third of ozonated sludge was mineralized via the recirculation treatment, and thereby the requirement of sludge mass to be treated was 3.3 times as much as sludge to be eliminated. Effluent TOC was slightly higher than under the conventional activated sludge process, indicating that refractory TOC was released from the sludge eliminated by treatment. The amount of released TOC corresponded to less than 2 weight % of eliminated sludge under recirculation rates below 30% of total biomass in the aeration tank in a day, but increased at higher recirculation rates. The operation costs associated with the process were estimated to be lower than those of conventional dewatering and disposal.

Sustainable sewage treatment plants — application of nanofiltration and ultrafiltration to a pressurized bioreactor

Based on earlier studies and experiences the Institute of Sanitary Engineering, Water Quality and Waste Management at the University of Stuttgart has developed a new technology to purify wastewater to a quality suitable for reuse as industrial water. This technology consists of a pressurized bioreactor under 3 bar with an ultrafiltration unit for sludge retention coupled with a nanofiltration unit. The concentrate retained in the nanofiltration is fed back to the bioreactor resulting in an enhanced biodegradability and a better quality of the effluent. The flux through the ultrafiltration membrane ranged from 100 to 200 l/(m3 h); that through the nanofiltration membrane was around 0.05 l/(m3 h). While attaining a very high biomass concentration (20 to 40 kg MLVSS/m3) in the bioreactor the excess sludge production is remarkably small with 0.7 to 1.0% of the influent COD.

An innovative approach to reduce excess sludge production in the activated sludge process

A new process has been developed to reduce excess sludge production, in which both excess sludge digestion and wastewater treatment are conducted simultaneously in the same aeration tank. The ozonation enhances biological degradation of the activated sludge, which is decomposed in a subsequent biological treatment. A considerable amount of biomass is mineralized biologically in proportion to the amount of recirculated biomass from the ozonation stage to the biological stage. It was observed that the amount of excess sludge is reduced to nearly zero when 1.2 kg/m3-aeration tank volume of biomass is recirculated in a day from the biological stage to the ozonation stage at a BOD loading of 1.0 kg/m3/d. A biomass concentration of 4200 mg/L was maintained at 1.0 kg-BOD/m3/d without drawing excess sludge for 6 weeks of experimental period under ozone dose of 0.05 g-O3/g-SS and recirculation rate at 0.3 d−1. Only a limited difference in the effluent quality was observed between the new process and the conventional activated sludge process.

Feasibility of Anaerobic Pre-Treatment for the Effluents from Hardboard and Laminated Board Industry

This paper presents results of a comparative evaluation of the feasibility of anaerobic pre-treatment for the effluents from a hardboard and laminated board industry. The problems associated with the treatment of hardboard mill wastewaters using activated sludge process have been observed as relatively low COD removal, high energy cost and high amounts of excess biological sludge production. The purpose of the study is to evaluate the effectiveness and feasibility of the anaerobic pre-treatment to reduce the excess biological sludge production and energy consumption in the aerobic stage. Anaerobic treatability studies were carried out by a lab-scale upflow anaerobic sludge blanket reactor (UASBR) with an effective volume of 10 litres. The UASBR was operated for a wide range of organic and hydraulic loadings during the experimental studies of more than 4 months. Anaerobic treatability studies have shown that a COD removal of 60% is possible at organic loading rates of 5-11 kg COD/m3.d. Anaerobic treatability study results have shown that 70 percent reduction both in the excess sludge production and in energy for aeration are possible by applying anaerobic pre-treatment. Considering these findings, it was demonstrated that anaerobic pre-treatment is a feasible option for waste management of the hardboard mill effluents.

Activated sludge process performance using a multistage tower aeration tank

This study's objective was to clarify both experimentally and theoretically whether a vertical multistage tower aeration tank system is advantageous as compared with a completely mixed system, particularly with respect to purification efficiency, sludge settleability, and excess sludge production. In comparing the two systems: (1) purification efficiency in the multistage tower aeration system with partial fluid mixing with a large Peclet number was higher than in a corresponding completely mixed system for all applied organic loadings; (2) the multistage tower aeration system had some definite advantages with respect to sludge settleability and excess sludge production; (3) the activated sludge system's higher performance with partial fluid mixing was shown quantitatively with the axial dispersion model in conjunction with growth kinetics which involved rapid uptake such as biosorption and subsequent oxidative biodegradation processes of organic substances.

Evaluation of the Impact of Return Flows on the Operation of a Wastewater Treatment Plant

A systemic analysis of a wastewater treatment plant quickly reveals that return flows from the sludge treatment line affect the water treatment process. These return flows have a maximum value that is characteristic of a steady state of plant operation and the effects are detectable at several levels. It can, for example, influence the operation of the clarifier indicated by changes in the Sludge Volume Index, induce anaerobiosis in the separation works that stems from excess sludge production or decrease the dewatering of sludges. All these observations lead to the conclusion that the sludge treatment line should be designed that it could take up production overloads. It was also found that it is indispensible to monitor the water treatment line operation step by step in order to detect rapidly any drift of the operating parameters from their normal values.

Anaerobic Treatment of Domestic Sewage and Wastewater

Recent UASB-pilot plant and full scale experiences show that anaerobic pre-treatment is feasible for sewage and domestic wastewaters in tropical regions. This offers developing countries the unique opportunity to become completely self-sufficient in this respect. UASB-systems can be applied at very small and at very big scale, i.e. for off-site and (house and community) on-site treatment. Results obtained in pilot plant and full scale installations demonstrate that these systems are quite effective in removing and stabilizing organic pollutants at liquid detention times as low as 4 hrs, i.e. a BOD reduction up to 75-95 % can be achieved, while an almost complete treatment can be accomplished by combining the method with a small aerobic lagoon. Instead of consuming high grade energy, these systems produce energy. Moreover the excess sludge production is very low, while also the dewatering characteristics of the sludge are excellent.

Sludge Utilization and Reduction Experiences in the Pulp and Paper Industry

This paper presents pilot-scale experiences on utilization of primary sludges from an integrated pulp and paper mill in the manufacturing of hardboard. The results have shown that the primary sludge added in 1/4 ratio yields the best result considering quality and operating aspects for the hardboard production. Lab-scale anaerobic treatability studies were carried out by an upflow anaerobic sludge blanket reactor (UASBR) with a volume of 10.351 using settled effluents from the related industry as the feed. Anaerobic treatability study results have shown that such an application prior to existing activated sludge system may provide 70 percent reduction in energy requirement for aeration and about 65 percent net reduction in excess biological sludge production.

Performance of the 5 MLD UASB Reactor for Sewage Treatment at Kanpur, India

The present paper describes the performance of a 1200 m3 UASB reactor treating 5000 m3 municipal waste water per day at Kanpur, India. The reactor was constructed in three parallel compartments of 600, 300 and 300 m3 respectively to study the influence of various design and operational parameters on process performance. The 600 m3 compartment is the reference unit. In one of the 300 m3 compartments the effluent overflow gutters are provided with baffles to prevent overflow of floating material. The other 300 m3 compartment is provided with a double amount of inlet pipes at the bottom of the reactor to assess the effect of a more dense distribution of the influent. The start-up of the reactor was carried out without addition of seed sludge since the influent contained the necessary seeding material. Data collected over a period of twelve months showed that there was an average reduction in COD, BOD, and TSS concentrations of respectively 74, 75, and 75 % at a hydraulic retention time of 6 hours. Excess sludge production was 0.2 kg TSS/m3 waste water, having 60-70 percent ash. It could be dried in six days on open sludge drying beds. The biogas yield was 0.05 to 0.10 m3/kg COD removed. The gas had 75 to 80 percent methane. Also during winter time the treatment efficiency and process stability remained good. The compartment provided with baffles showed better removal efficiencies for COD, BOD, and TSS than the other compartments. A double number of inlet points did not show an increase in removal efficiency. A density of one inlet point per 3.7 m2 is sufficient to provide a properly distributed influent in the reactor.

Sewage treatment under conditions of balancing microbial growth and cell decay with a high concentration of activated sludge supplemented with ferromagnetic powder

In order to decrease excess sludge production, the treatment of synthetic sewage was carried out under conditions of balancing microbial growth and cell decay with a high concentration of activated sludge supplemented with a ferromagnetic powder (magnetic sludge). The leakage of magnetic sludge was prevented by the combined treatment of magnetic separation and gravitational sedimentation. CODCr, removal and effluent suspended solids concentrations were maintained at 85–94% and 5–24 mg l−1, respectively, over 30 d. Meanwhile, mixed-liquor volatile suspended solids concentration in the aeration tank remained virtually constant and excess sludge was not produced.

微生物支持体「バイオループ」を用いた下水処理

Bioloop filter (braided cord made of vinylidene chloride thread) was used for the biological treatment of sewage. In this study, the aeration tank (1.8 m3) was divided into 4 portions across the flow direction and bioloop filter (30 or 45 m·m-3) was submerged in each tank. Amount of biomass attached to the filter was determined after 60 to 100 days operational periods and were regarded as mixed liquor suspended solids (MLSS).The amount of biomass attached on the filter was ranged from 0.4 to 0.6 kg·m-3 (bioloop 30 m·m-3) and from 0.4 to 0.8 kg·m-3 (bioloop 45 m·m-3). Acoordingly, BOD loadings in this study were kept in the range from 0.15 to 0.35 kg-BOD·kg-MLSS-1·d-1. Effluent BOD was stabilized with the decrease of BOD loading and effluent containing BOD<5 mg·l-1 was obtained even in winter season at a loading under 0.24 kg-BOD·MLSS-1·d-1. Though dissolved oxygen concentration in bulk liquid was maintained at suficiently high level, biomass contained sulfide ion upto 0.4 mg-S2-·g-biomass-1. As a result of anaerobic degradation inside the biomass, the excess sludge production ratio of this process was less than that of conventional activated sludge.

On the Problem of “Two-Phase” Activated Sludge

Steady-state models are presented to describe the wastewater treatment process in two activated sludge systems. One of these makes use of a single complete-mix reactor; the other one involves two complete-mix reactors arranged in series. The in-series system is equivalent to what is known as the “two-phase” activated sludge, a concept which is now being launched throughout Poland in conjunction with the PROMLECZ technology under implementation. Analysis of the mathematical models has revealed the following: (1) treatment efficiency, excess sludge production, energy consumption, and the degree of sludge stabilization are identical in the two systems; (2) there exists a technological equivalence of “two-phase” sludge with “single-phase” sludge; (3) the “two-phase” system has no technological advantage over the “single-phase” system.

Enlargement of Sewage Treatment Plants for Nitrification by Use of Subsequent Rotating Biological Contactors

The surface-specific nitrification performance of Rotating Biological Contactors (RBC) has been improved considerably by the use of new techniques. Maximum nitrification rates of 3.3 g/m2×d (10 °C) were achieved, which cannot be approached with any other known procedure. This high oxidation efficiency is achieved with Rotating Biological Contactors by using two new measures:A large degree of separation of carbon degradation and nitrogen oxidation through filtration of the waste water before the subsequent nitrifying RBC stage, whereby the environmental conditions for the nitrifying bacteria are improved, especially in respect of an optimised supply of oxygen.Periodic reversal of the direction of flow in the RBC stage which is constructed as a cascade. This measure allows the attainment of a uniform thickness of nitrifying bacteria growth in all the compartments, which has a particularly advantageous effect in the case of unavoidable surges in load. Because of the low production of excess sludge in the nitrification stage, a subsequent stage for separating solids is not necessary. In this paper experiences with the upgrading of a full scale rotating biological contactor plant and the measuring results from an enlarged oxidation pond installation are summarized.