Transfer of organic matter between wastewater and activated sludge flocs

The organic matter of wastewater was fractionated into settleable (i.e., particulate) and non-settleable (i.e., colloidal + soluble) fractions by settling followed by 0.22 micron filtration. Particulate, colloidal and soluble proportions were found to be relatively constant (45, 31 and 24% of the total COD, respectively). Transfer of soluble fraction always occurred from the wastewater to the activated sludge flocs, whereas bidirectional transfer occurred for the colloidal fraction. The transfer of soluble and colloidal matter reached a steady state after 40 min-mixing and 20 min-mixing, respectively. Desorption of a part of the colloidal organic matter pre-sorbed on the activated sludge flocs was evidenced. The biosorption capacity of activated sludge was around 40-100 mgCODg-1TSS. The biosorbable fraction of wastewater represented on average 45% of the non-settleable fraction.     

On the stability of activated sludge flocs with implications to dewatering

It was shown that Ca²⁺ can be extracted from activated sludge from a plant with biological nitrogen and phosphorus removal either by an ion exchange process, where H⁺, Na⁺, K⁺ or Mg²⁺ served as counter ions, or by chelation by EGTA. The extraction of Ca²⁺ led to an increase in the number of small particles and subsequently an increase in the specific resistance to filtration.

It was argued that approx. half of the total Ca²⁺ pool was associated with the exopolymers and thereby formed an alginate-like gel, which constituted the backbone of the floc structure. This notion was further emphasized by the observation that addition of Cu²⁺ which, along with releasing Ca²⁺, improved the dewaterability—probably because of the Cu²⁺ ions ability to maintain the three-dimensional structure of the exopolymers.     

Flocculation of activated sludge flocs by stimulation of the aerobic biological activity

Activated sludge flocs are known to deflocculate under short-term anaerobic conditions, but little is known about possible reflocculation under subsequent aerobic conditions. When activated sludge flocs from two wastewater treatment plants deflocculated under anaerobic conditions with well-defined shear conditions, they could be almost, but not completely, reflocculated by aeration for 1-2 h under the same shear conditions. If the biological activity was reduced by adding azide, chloramphenicol or by decreasing the temperature, no or only very little reflocculation took place. This indicated that the reflocculation was under direct or indirect microbial control. Only a small part of the reflocculation was due to improved flocculation properties obtained by oxidation of Fe(II) to Fe(III), which is a better flocculant. Fe(II) was produced under the anaerobic conditions by microbial iron reduction, and it was oxidized to Fe(III) within less than one hour after the aeration was started. However, by comparing two different sludges with different capabilities for iron reduction, iron oxidation and responses to substrate addition, it was found that the aerobic biological activity most likely was of greatest significance for the observed reflocculation and floc formation under aerobic conditions. This was further supported by adding organic substrates (glucose or ethanol) during the aerobic reflocculation phase, which promoted reflocculation. However, some substrates had the opposite effect (acetate and lactate), where a deterioration of the reflocculation was observed, probably due to different responses from different groups of microorganisms in the sludges.     

Structure of conditioned sludge flocs

Free settling tests, small-angle light scattering, microtome-slicing techniques, and confocal laser scanning microscopy were performed to examine how the cationic flocculation or freezing and thawing affected the floc structure. The floc size, internal pore size, mass fractal dimensions determined from free-settling test or small angle light scattering test, aeral porosity, boundary fractal dimension and Sierpinski carpet fractal dimension of pore boundary from 2D slices, and the volume porosity, compactness, and the pores' box-counting fractal dimension from 3D reconstructed image, were estimated and compared. Cationic flocculation would produce large flocs with internal pores of shape resembling a long "tube" with rough surface. Freezing and thawing would produce flocs with internal pores with lower aspect ratio and a smoother boundary.     

Characterization of activated sludge flocs by confocal laser scanning microscopy and image analysis

In this study we present a new approach to determine volumes, heterogeneity factors, and compositions of the bacterial population of activated sludge flocs by 3D confocal imaging. After staining the fresh flocs with fluorescein-isothiocyanate, 75 stacks of images (containing approx. 3000 flocs) were acquired with a confocal laser scanning microscope. The self-developed macro 3D volume and surface determination for the KS 400 software package combined the images of one stack to a 3D image and calculated the real floc volume and surface. We determined heterogeneity factors like the ratio of real floc surface to the surface of a sphere with the respective volume and the fractal dimension (D(f)). According to their significant influence on floc integrity and quality, we also investigated the chemical composition of flocs and quantified their bacterial population structure by using group-specific rRNA-targeted probes for fluorescence in situ hybridization. By a settling experiment we enriched flocs with poor settling properties and determined the above-mentioned parameters. This approach revealed shifts in floc volume, heterogeneity, and bacterial and chemical composition according to the settling quality of the flocs.     

Test for assessing shear sensitivity of activated sludge flocs: a feasibility study

Sludge deflocculation can cause, like excessive growth of filamentous bacteria, activated sludge wastewater treatment failure. Yet, unlike the latter cause, there exists no widely accepted tool to assess the flocculation level of activated sludge and to predict sludge deflocculation. In this study, a test procedure is proposed to assess the sensitivity of activated sludge flocs to shear. The test consists in subjecting sludge sample to a shear treatment followed by a sludge volume measurement (SV(30)), with reference to a control. The ratio (%) obtained by dividing the SV(30) of the sheared sample by the SV(30) of the control is used to express the shear sensitivity. In a first series of experiments using two types of sludge, the test was shown to be able of ranging sludge samples in a correct order of shear sensitivity. Applying this so-called shear-SV test procedure to a series of sludge samples allowed to distinguish amorphous sludges (SV(30) ratio> or =100%) from normal sludges (SV(30) ratio<100%). The test was shown to be repeatable and simple. It requires only basic laboratory equipment and implies minimum cost. It can be used as framework for the assessment of activated sludge shear sensitivity in practice.     

The activated sludge process—3 single sludge denitrification

The general aerobic bi-substrate active-site death-regeneration activated sludge model including nitrification of Dold et al. (Prog. Wat. Technol.12, 47–77, 1980) is extended to include the kinetic behaviour of the denitrification process in single sludge systems. The extension requires a change in the value of only one of the kinetic constants (K′_mp) in the expression for the particulate substrate utilization rate when the environment becomes anoxic. The extended model simulates very closely the response of the multi-reactor nitrification-denitrification process configurations under both constant and cyclic flow and load conditions. Under constant flow and load conditions, the denitrification response predicted can be reduced to that approximated by a zero order reaction dN/dt = ?KX_a with two rates in the primary and one in the secondary anoxic reactor respectively.     

Structural, physicochemical and microbial properties of flocs and biofilms in integrated fixed-film activated sludge (IFFAS) systems

Integrated fixed-film activated sludge systems (IFFAS) may achieve year-round nitrification or gain additional treatment capacity due to the presence of both flocs and biofilms, and the potential for multiple redox states and long solids retention time. Flocs and biofilms are distinctive microbial structures and characterization of the physicochemical and structural properties of these may provide insight into their respective roles in wastewater treatment and contaminant removal in IFFAS. Flocs and biofilms were examined from five different pilot media systems being evaluated for potential full scale implementation at a large municipal wastewater treatment plant. Flocs and biofilms within the same system possessed different surface characteristics; flocs were found to have a higher negative surface charge (−0.35 to −0.65 meq./g VSS) and are more hydrophobic (60%-75%) than biofilms (−0.05 to −0.07 meq/g VSS; 19-34%). The EPS content of flocs was significantly higher (range of 2.1-4.5 folds) than that of biofilms. In floc-derived extracellular polymeric substances (EPS), protein (PN) was clearly dominant; whereas in biofilm-derived EPS, PN and polysaccharide (PS) were present in approximately equal proportions. Biofilm EPS had a higher proportion of DNA when compared to flocs. Biofilm growth was preferential on the protected internal surfaces of the media. Colonization of the external surfaces of the media was evident by the presence of small microcolonies. The structural heterogeneity of the biofilms examined was supported by observed differences in biomass content, thickness and roughness of biofilm surface. The biofilm on the interior surface of media was found to be patchy with clusters of cells connected by an irregular arrangement of interconnecting EPS projections. Biofilm thickness ranged between 139 μm and 253 μm. The pattern of oxygen penetration is expected to be complex. Nitrifiers and denitrifiers were predominantly associated with the biofilms, and the latter were found to be dispersed throughout the film and arranged in micro-clusters, suggesting partial oxygen penetration.     

Experimental and numerical investigations of sedimentation of porous wastewater sludge flocs

The paper studies the properties and sedimentation characteristics of sludge flocs, as they appear in biological wastewater treatment (BWT) plants. The flocs are described as porous and permeable bodies, with their properties defined based on conducted experimental study. The derivation is based on established geometrical properties, high-speed camera data on settling velocities and non-linear numerical model, linking settling velocity with physical properties of porous flocs. The numerical model for derivation is based on generalized Stokes model, with permeability of the floc described by the Brinkman model. As a result, correlation for flocs porosity is obtained as a function of floc diameter. This data is used in establishing a CFD numerical model of sedimentation of flocs in test conditions, as recorded during experimental investigation. The CFD model is based on Euler-Lagrange formulation, where the Lagrange formulation is chosen for computation of flocs trajectories during sedimentation. The results of numerical simulations are compared with experimental results and very good agreement is observed.     

The kinetics of protein removal by activated sludge

The removal of bovine serum albumin (BSA) by BSA acclimated activated sludge took place in two stages. Removal during stage 1 was attributed to adsorption. The removal of BSA during stage 2 was slower than stage 1. There was never an accumulation of protein degradation products and removal was accompanied by an increase in the metabolic activity of the activated sludge. The removal of BSA per unit biomass during stage 2 was influenced by both the concentration of BSA and the concentration of activated sludge At high concentrations of BSA, removal was preceded by a lag phase. The rate of removal during stage 2 was constant. Within limits the rate of removal of BSA was related to the substrate to biomass ( ) ratio of the system and to the concentration of BSA expressed per unit biomass remaining in equilibrium following stage 1 of BSA removal.     

Variation of sludge volume index with activated sludge characteristics

Activated sludge samples from 2 laboratory units and 12 sewage treatment plants were examined to determine the effect of filamentous microorganisms, floc size and suspended solids concentration on SVI. An attempt was also made to correlate SVI to zone settling velocity. At a suspended solids concentration range of 700–4800 mg 1⁻¹ there was no effect of filamentous microorganisms at filament length concentrations below 10⁷μm (mg SS)⁻¹. However, when it was over 10⁷ μm (mg SS)⁻¹ SVI increased sharply with increasing concentrations of filamentous microorganisms.At all suspended solids concentrations examined SVI varied with floc size at filament length concentrations below 10 μm (mg SS)⁻¹. But, at filament length concentrations higher than this level, no effect of floc size on SVI was observed.The effect of suspended solids concentration on SVI was examined at different levels of filament lengths. It was found that the shape of SVI-suspended solids concentration curve varied with the level of filament lengths. A well defined relationship was found between SVI and zone settling velocity at all suspended solids concentrations examined.     

The effects of organotin on the activated sludge process

Organotin compounds which find increasing use in marine antifouling paints may be present in the discharge from dry dock operations. This investigation was aimed at determining the effect of such wastewater when discharged to a municipal activated sludge treatment plant. Experiments were conducted using a Warburg respirometer and continuous flow bench-scale activated sludge systems. The results showed that unacclimated biological cultures can be inhibited by tributyl tin oxide (TBTO^™) concentrations as low as 25 μgl⁻¹. However, TBTO doses of over 8000 μgl⁻¹ can be tolerated by a well acclimated culture. Continuous loading of up to 1000 μgl⁻¹ TBTO had no effect on organic removal in activated sludge systems. However, an adverse effect on sludge settleability was noticed at 100 μgl⁻¹ TBTO. Shock loadings of 500 and 1000 μgl⁻¹ TBTO had no effect on soluble organic removal but resulted in impaired settling and higher effluent suspended solids. The LC₅₀ of TBTO to the fathead minnow was estimated at 45–200 μgl⁻¹. The toxicity was reduced considerably by activated sludge treatment.     

The adsorption of bovine serum albumin by activated sludge

The rapid removal, from suspension, of between 2–% of bovine serum albumin (BSA) by BSA acclimated activated sludge was attributed to adsorption. The extent of adsorption varied with the substrate to biomass (s/b) ratio. The concentration of BSA adsorped was influenced by both the concentration of BSA and the concentration of activated sludge. The experimental data did not conform to the calssical adsorption equations of Langmuir (J. Am. chem. Soc.40, 1361–1403, 918) or Freundlich (Colloid and Capillary Chemistry, Methuen, London, 1926) but to a newly developed equation, the activated sludge adsorption equation (ASAE). This new equation was tested and proven by experimental data and by data obtained independently by Banerji et al. (J. Wat. Pollut. Control Fed.40, 161–173, 1968) who investigated starch removal by activated sludge. Following the development of the ASAE, it was found possible to express both the concentration of BSA adsorbed per unit weight activated sludge (m) and the concentration of BSA in equilibrium per unit weight activated sludge (C/b) as a function of the concentration of BSA added to the system per unit weight adsorbent (C_t). Thus adsorption could be expressed as a function of the substrate to biomass (s/b) ratio.     

Respirometric measurements on activated sludge

The parameters required to describe the metabolism of activated-sludge according to the equations of Michaelis and Menten or Monod can be determined by simple batch experiments. For the sake of clarity one should distinguish between “long-run” batch experiments with a period of unlimited growth, like the BOD determination, and “short-run” experiments with a very small relative cell growth. In this paper the possibilities of a simple short-term oxygen demand test are examined. This method is expected to be applied in practice for process control, viz. continuous monitoring of influent BOD, measuring the concentration of viable sludge bacteria and for measuring toxicity.     

The influence of sludge age on heavy metal removal in the activated sludge process

In a laboratory simulation of the activated sludge process ten heavy metals were added continuously to the system which was allowed to equilibrate at six sludge ages between 3 and 18d. Cobalt, manganese and molybdenum removals were poor and were unaffected by changes in the sludge age. The highest removal efficiencies for the other metals occurred at the 15d sludge age. Chromium (trivalent) and cadmium had the highest removal efficiencies, typically greater than 50%. The behaviour of the majority of the metals which were removed to a significant extent was related to one of the parameters influenced by sludge age, i.e. mixed liquor suspended solids, effluent suspended solids or effluent chemical oxygen demand. The metals which were poorly removed showed little affinity for the activated sludge, while most metals exhibited maximum specific uptake by the mixed liquor at a sludge age of 9–12d. However, the affinity of silver for the mixed liquor continued to increase as the sludge age increased to 18d. An affinity series, based on an arbitrary measure of the specific accumulation of metals by the mixed liquor, indicated that chromium, cadmium and silver were most readily adsorbed by the activated sludge.     

Co-conditioning and dewatering of chemical sludge and waste activated sludge

The conditioning and dewatering behaviors of chemical and waste activated sludges from a tannery were studied. Capillary suction time (CST), specific resistance to filtration (SRF), and bound water content were used to evaluate the sludge dewatering behaviors. Zeta potentials were also measured. Experiments were conducted on each sludge conditioned and dewatered separately, and on the sludge mixed at various ratios. Results indicate that the chemical sludge was relatively difficult to be dewatered, even in the presence of polyelectrolyte. When the waste activated sludge was mixed with the chemical sludge at ratios of 1:1 and 2:1, respectively, the dewaterability of chemical sludge improved remarkably while the relatively better dewaterability of the waste activated sludge deteriorated only to a limited extent. As the mixing ratios became 4:1 and 8:1, the dewaterability of the mixed sludge was equal to that of the waste activated sludge. The optimal polyelectrolyte dosage for the mixed sludge was equal to or less than that of the waste activated sludge. It is proposed that the chemical sludges act as skeleton builders that reduce the compressibility of the mixed sludge whose dewaterability is enhanced. Bound water contents of sludge decreased at low polyelectrolyte dosage and were not significantly affected as polyelectrolyte dosage increased. Advantages and disadvantages of co-conditioning and dewatering chemical sludge and waste activated sludge were discussed.     

The activated sludge process—IV: Application of the general kinetic model to anoxic-aerobic digestion of waste activated sludge

This paper discusses the application of the general activated sludge model as set out by Dold et al. (Prog. Wat. Technol.12, 47–77, 1980) and extended by Van Haandel et al. (Wat. Res.15, 1135–1152, 1981), to anoxic-aerobic digestion of waste activated sludge. The laboratory scale experimental investigation comprised a 6 day sludge age activated sludge process, the waste sludge from which was fed to a number of digesters operated as follows: single reactor flow-through digesters at 4 or 10 days sludge age (retention times) under aerobic or anoxic-aerobic conditions (with 1.5 and 4 h cycle times) and 3-in-series flow-through aerobic digesters each with 4 days sludge age; all digesters were fed draw-and-fill wise once per day. The general kinetic model simulated accurately all the experimental data without the need to change the values of the kinetic constants. Both theoretical simulations and experimental data indicate that (i) the rate of volatile solids destruction is not affected by the incorporation of anoxic cycles and (ii) the specific denitrification rate constant in a digester is about two-thirds of that in the secondary anoxic reactor of the single sludge activated sludge system; this allows definition of a fourth denitrification rate constant K₄ for the anoxic-aerobic digester with K₄T = 0.046(1.029)^(T-20) mg(NO₃-N) (mgAVSS d)⁻¹, a constant independent of sludge age. An important consequence of (i) and (ii) above is that the denitrification can be integrated readily into the steady state digester model of Marais and Ekama (Wat. SA2, 163–200, 1976) and used for design purposes.     

Benthal stabilization of waste activated sludge

Laboratory studies were performed on the stabilization of waste activated sludge solids in a deposit formed at the bottom of an aerated water column. Conclusions reached were as follows: (1) As long as dissolved oxygen is maintained in the water column, the benthal process is stable and odor free. (2) At temperatures within the range of 20 ± 1°C, an average benthal stabilization rate of 79.4 g m⁻² d⁻¹ (biodegradable solids) can be attained if solids are not limiting. (3) Under conditions stated in conclusion (2), as much as 63% of total carbon stabilization can occur via methane formation. (4) A year-old benthal deposit of waste activated sludge solids can be expected to have a solids percentage of about 2.3%