Dewatering model for optimal operation of sludge treatment wetlands

Sludge treatment wetlands (STW) are used as a dewatering technology in some European countries since the 80’s. Although the efficiency of this technology in terms of sludge dewatering and mineralisation is well known, design and operation parameters are yet to be standardised. The aim of this study is to develop a mathematical model capable of predicting the water loss with time, in order to optimise the feeding frequency enhancing sludge dewatering and expanding the lifespan of the system. The proposed model is validated with experimental data from one pilot and two full-scale STW. The scenarios considered indicate that the optimum feeding frequency decreases with the sludge layer height. In this way, systems with a sludge layer of 20 cm, 40 cm and 80 cm (corresponding to 2, 4 and 8 years of operation), should be fed every 2.5, 10 and 30-40 days, respectively. On the other hand, evapotranspiration (ET) has no effect on the feeding frequency, although it does increase the sludge dryness from 25% to 45% (for ET of 2.5 and 14.5 mm/d in the case of 20 cm of sludge height). According to the model output, the sludge loading rate is determined as a function of evapotranspiration, feeding frequency and sludge height.     

Resuspension and settling of helminth eggs in water: Interactions with cohesive sediments

Helminth parasite eggs in low quality water represent main food safety and health hazards and are therefore important indicators used to determine whether such water can be used for irrigation. Through sedimentation helminth eggs accumulate in the sediment, however resuspension of deposited helminth eggs will lead to increased concentration of suspended eggs in the water. Our study aimed to determine the erodibility (erosion rate and erosion threshold) and settling velocity of Ascaris and Trichuris eggs as well as cohesive sediment at different time points after incorporation into the sediment. Cohesive sediment collected from a freshwater stream was used to prepare a sediment bed onto which helminth eggs were allowed to settle. The erodibility of both sediment and helminth eggs was found to decrease over time indicating that the eggs were incorporated into the surface material of the bed and that this material was stabilized through time. This interaction between eggs and bulk sediment was further manifested in an increased settling velocity of suspended eggs when sediment was present in the suspension as compared to a situation with settling in clean water. The incorporation into the sediment bed and the aggregation with sediment particles decrease the mobility of both helminth egg types. Our findings document that helminth eggs should not be viewed as single entities in water systems when modelling the distribution of eggs since both erodibility and settling velocity of eggs are determined by mobility of the sediment present in the water stream. Recalculation of the erosion threshold for helminth eggs and sediment showed that even at relatively low current velocities i.e. 0.07-0.12 m s⁻¹ newly deposited eggs will be mobile in open irrigation channels. These environmental factors affecting resuspension must be taken into account when developing models for sedimentation of helminth eggs in different water systems.     

Predictive model for treatment of phenolic wastes by activated sludge

The current investigation was made to determine whether a model using cell recycle concentration as a major control parameter is applicable to inhibitory substrates such as phenol. Based upon preliminary studies, the Haldane equation was selected to relate specific growth rate and substrate concentration. Seven long-term pilot plant runs were made at different growth rates. Effluent and control parameters were measured frequently. The maintenance constant was obtained from the pilot plant data. The three Haldane equation constants and the true cell yield were estimated from pilot plant and batch data.The model gave satisfactory predictions with respect to biomass production (X and X_w) and effluent COD, based on the ΔCOD method. The predicted phenol concentration, although very low, was higher than the minute levels observed in the pilot plant effluent (±0.1 mg l⁻¹); possible reasons not directly related to growth are explored, e.g. retention of unmetabolized phenol in the sludge.     

城市污水处理厂污泥样品检测前样品的预处理

通过选取污泥作为研究对象,在大量实验的基础上,针对污泥样品检测前的预处理进行了分析总结,并对污泥样品的分解与浸提作了探讨,对于准确测定污泥的组分含量具有一定的实用价值。     

Development of an advanced biological treatment system applied to the removal of nitrogen and phosphorus using the sludge ceramics

To develop a method of forming lake sediment into sludge ceramics with porosity and good biological adhesion for use as a medium for microorganisms in wastewater treatment, a study of the effects of forming conditions was conducted by adjusting the water content of sludge and compounding some additives. By adjusting the water content of the raw material at the kneading/pelletizing step to 40–42% and adding 3% waste glass to the raw materials to make up for the lack of flux, a sludge ceramic with a density in terms of specific gravity of saturated surface dry aggregate of about 1400 kg m⁻³ was formed. In addition, to develop a small-scale wastewater treatment system capable of removing nitrogen and phosphorus, a sludge ceramic was applied as a medium for biological filtration. The results indicated that the BOD removal nitrification rate were superior to those of conventional ceramic media, reached at 95.3% and 87.4%, respectively. The introduction of iron electrolysis resulted in high treatment performance achieving BOD levels of 10 mg L⁻¹ or less, T-N of 10 mg L⁻¹ or less and T-P of 1 mg L⁻¹ or less.     

Oxone/Co oxidation as an advanced oxidation process: Comparison with traditional Fenton oxidation for treatment of landfill leachate

In this paper, the application of Fenton and Oxone/Co²⁺ oxidation processes for landfill leachate treatment was investigated. The removal of the chemical oxygen demand (COD), suspended substances (SS) and the color of the landfill leachate by Fenton oxidation to that by Oxone/Co²⁺ oxidation were compared under optimal operational conditions. For Fenton oxidation process, the optimal conditions were determined as: [H₂O₂] = 80 mmol L⁻¹, [H₂O₂]/[Fe²⁺] = 2.0, initial pH = 2.5, reaction temperature = 37.5 ± 1 °C, reaction time = 160 min, number of stepwise addition = 3. Under the given conditions, 56.9% of the COD removal efficiency was achieved, but the SS and the color of the treated landfill leachate increased due to the generation of a large quantity of ferric hydroxide sludge. In reference to Oxone/Co²⁺ oxidation process, the optimal conditions were determined as: [Oxone] = 4.5 mmol L⁻¹, [Oxone]/[Co²⁺] = 10⁴, pH = 6.5, reaction temperature = 30 ± 1 °C, reaction time = 300 min, number of stepwise addition = 7, the COD, SS and the color removal efficiencies were 57.5, 53.3 and 83.3%, respectively. From this work, it can be concluded that Oxone/Co²⁺ oxidation process demonstrated higher degradation efficiencies of the COD, SS and color for landfill leachate treatment than that by Fenton oxidation process. It also suggested that Oxone/Co²⁺ oxidation process could be considered as one of the most promising technologies for practical applicability to treat landfill leachate in large scale. For further improving the efficiency of Oxone/Co²⁺ oxidation process, we proposed that combination of it with other technologies in future such as ultraviolet, ultrasound and biological methods.     

Calibration and validation of an ASM3-based steady-state model for activated sludge systems--part I: Prediction of nitrogen removal and sludge production

The steady-state model from Siegrist and Gujer (1994) which can be used for the design and optimisation of nitrogen-removing activated sludge plants is applied to the stoichiometrics and kinetics of a validated Activated Sludge Model No. 3. It considers the wastewater composition, the effect of the electron acceptor on the average sludge production, the oxygen input into anoxic volumes, denitrification in the secondary clarifier, the temperature and various operating conditions. The organic substrate for denitrification originates from readily degradable substrate from the influent, from the hydrolysis of slowly degradable particulate substrate along the activated sludge plant and from the endogenous respiration of the biomass. The model is calibrated and validated with data from long-term full-scale and pilot-plant experiments for Swiss municipal wastewater. The most sensitive parameters as well as the uncertainty of the model prognosis for various COD-to-nitrogen ratios from inlet water and anoxic volume fractions were calculated with the aid of sensitivity analyses and Monte-Carlo simulations. Excel spreadsheets of the model for different flow schemes are available from the corresponding author.     

Powdered activated carbon in an activated sludge treatment plant

Addition of powdered activated carbon (PAC) to the aeration basin of an activated sludge treatment plant fed with dye-works waste waters increases the purifying capacity of the plant: removal efficiency rises from 55.8 to 75.6% (COD) and from 78 to 98.5% (BOD₅) and the nitrification-denitrification capacity of the system also increases.The sludge growth parameters and the kinetic constant of biological oxidation were determined on the plant with and without PAC. The addition of PAC decreased overall sludge growth rate and the auto-oxidation factor, but increased the biological removal rate of the substrate by about one order of magnitude.     

Development of a biological filtration model applied for advanced treatment of sewage

A mathematical model of biological filtration process is developed in this paper. A biological filtration process has advantages that filtration action and biological activities are combined in a single reactor with aid of filter media. Both physical and biological functions are incorporated in this developed model to simulate both mechanisms. Backwashing is expressed by the assumption that a mean captured solids concentration is input as data, and a captured solids concentration is kept at that value during each filtration run. The developed model is applied to explain the experimental performance with biological filtration reactors, in which batch cultivation of autotrophic bacteria and continuous treatment of actual sewage are carried out. Its applicability is discussed by comparing the simulated results with the experimental data. This model can favourably estimate maximum accumulation of autotrophic bacteria on the medium in batch cultivation, long-term treatment performance in continuous treatment, details of water quality profiles through the filter bed, and biomass. Required hydraulic retention time for nitrification and an appropriate recirculation ratio in a winter season are discussed with this model. This model predicts that a HRT of 1.1 h or above is required to achieve nitrification with remaining NH4(+)-N of less than 1 mgN/L and that an appropriate recirculation ratio is 2-3.     

Moringa oleifera seeds as a flocculant in waste sludge treatment

In this study the results of a laboratory based investigation to determine the effectiveness of Moringa oleifera seeds as a flocculant for activated sludge treatment are presented. Waste sludge samples are activated sludge from Taman Dr. Tun. Ismail Activated Sludge Wastewater Treatment plant, Kuala Lumpur, Malaysia. Moringa oleifera seed was applied as dry powder (shelled blended), solution (shelled blended), and solution (shelled blended oil extracted). Results of vacuum filtration studies showed that residual sludge volume of up to 11%, 20% and 45% for shelled blended oil extracted, shelled blended, and control (no Moringa applied), respectively, was achievable. Sludge volume reduction of up to 67% was achievable using gravity settling for shelled blended solution and 48% for shelled blended oil extracted respectively compared to the control. The shelled blended category applied in powder form performed the same as the solution of shelled blended but performed better than the shelled blended oil extracted category in gravity settling. The specific cake resistance of sludge treated with shelled blended Moringa oleifera averaged 2.5 x 10¹²m/kg at 4000mg/l dosage. Vegetable oil from the shelled Moringa oleifera of up to 30% was obtained as a by product.     

Using alum sludge for clay brick: an Irish investigation

Throughout the world, alum sludge is dewatered and the resultant cakes are discarded in landfill. This paper reports a study to investigate the possible incorporation of alum sludge as a partial replacement for clay in clay brick manufacturing. It is the first study of this problem in Ireland. Alum sludge cakes and clay were separately dried, ground and sieved in preparation for making test specimens. Cylindrical clay bricks were made at different temperatures (800, 1000, 1100, 1200 °C), incorporating different percentages (0, 5, 10, 15, 20, 30, 40% by dry weight) of alum sludge. The bricks were then subjected to compressive strength test and submersion. Loss on ignition, water absorption and weight reduction were calculated. It was found that bricks containing up to 20% sludge, fired at 1200 °C, or containing 5% sludge and fired at 1100 °C have met the European and Irish Standards as set out by Eurocode 6 – ‘Design of Masonry Structures’. The firing temperature and the increase in sludge content affected the final clay-sludge brick colour. By increasing the proportion of alum sludge, compressive strength decreased and the final weight of the brick was reduced. Firing temperatures that are too high may result in damage to the bricks during firing. This study has demonstrated the promising potential and prospects for Irish dewatered alum sludge cakes in clay-sludge brick making.     

The microbiology of an activated sludge waste-water treatment plant chemically treated for phosphorus removal

Microbiological research was conducted on a dual, secondary wastewater treatment system which was part of The Pennsylvania State University wastewater treatment plant. Each aeration basin received identical wastewater which was the effluent from a high rate trickling filter. One of the aeration basins was dosed with aluminum sulfate for the purpose of phosphorus removal. The other aeration basin (control) was operated in the conventional manner without alum addition. Plate counts performed on combined chemical-biological sludge and control activated sludge revealed that a higher number of viable micro-organisms was contained in the chemical-biological sludge, but the magnitude of difference between the two sludges was significant depending on the culture medium employed. Results suggest the aluminum flocs formed in the chemical-biological treatment enmesh dispersed wastewater micro-organisms, some of which are qualitatively unlike those indigenous to natural activated sludge. The combined chemical-biological sludge contained significantly higher numbers of lipolytic, gelatinolytic, and thiosulfate oxidizing micro-organisms and, possibly, fewer nitrite oxidizing micro-organisms than did control activated sludge. Alum did not appear to affect flagellated protozoa in mixed liquor; however, amoeboid and ciliated protozoa were found less frequently in alum dosed than in control mixed liquor. The settled effluent from the combined chemical-biological aeration basin generally contained fewer total coliforms, fecal coliforms, and fecal streptococci than did counterpart control effluents.     

Viability of six species of larval and non-larval helminth eggs for different conditions of temperature, pH and dryness

Helminth eggs are the most difficult biological parasites to inactivate in wastewater and sludge. In developing countries, in particular, they are present in high concentrations and are the cause of many diseases that impact seriously on the human population. The process conditions for affordable inactivation are very variable, leading to different inactivation efficiencies. Temperature, dryness, pH and the developmental stage of the eggs must be taken into consideration to achieve complete inactivation. The objective of this research was to study the inactivation of six species of larval and non-larval helminth eggs of medical importance in developing countries under controlled conditions of temperature, pH, dryness and contact time. Results showed considerable differences in inactivation conditions among helminth eggs and a high level of resistance was confirmed for the eggs of Ascaris lumbricoides and Ascaris suum. The appropriate conditions for inactivation of all types of eggs were found by applying combinations of pH, temperature and dryness. At 45 °C it was possible to inactivate all species with a pH of 5.3 and 90% dryness within 6 days. If alkalization was applied, a pH of 12.7 was sufficient over 19 days at the same conditions of dryness and temperature. From these results it is proposed that both Ascaris spp. and Taenia solium may be used as indicators of biological contamination in wastewater and sludge.     

Mathematical model suggested for a carbon-activated sludge system

Integrated biological systems such as activated sludge-powdered activated carbon [PAC] combinations are used in ever increasing numbers. For conventional activated sludge systems excellent models existed which presumably were also suitable for application to a PAC-fed system. PAC addition enhances bacterial activity, adsorption of dissolved organics and settleability of activated sludge flocs in the final clarifier. Simple laboratory experiments can demonstrate the degree of improvement in a PAC-fed system over the conventional activated sludge one. When using the present model the evaluation of several kinetic constants stemming from simple experiments may help to predict the performance of large scale operation even under unsteady state conditions.     

Molecular characterization of the microbial community structure in two activated sludge systems for the advanced treatment of domestic effluents

Although activated sludge systems with enhanced biological phosphorus removal (EBPR) represent state-of-the-art technology for phosphate removal from wastewater it is still unknown which species of bacteria are responsible for the EBPR process. The aim of this study was to compare the bacterial composition of activated sludge from two laboratory plants with different modes of operation, anoxic/oxic- (EBPR, no nitrification) and Phoredox-system (EBPR, nitrification and denitrification) with particular emphasis on microorganisms responsible for EBPR process. In addition to fluorescence in situ hybridization (FISH), we applied further rRNA-based molecular techniques like terminal restriction-fragment length polymorphism analysis and comparative 16S rDNA analysis to yield additional information and to verify the results from FISH analysis, like e.g. for the identification of polyphosphate accumulating organisms (PAO). Despite the different modes of operation only minor differences in the bacterial composition were detected by FISH analysis based on the probes used in this study. In contrast T-RFLP analysis yielded characteristic community fingerprints for each of the investigated plants and comparative 16S rDNA analysis indicated highly diverse microbial communities in both plants suggesting substantial differences in the microbial structure. The results obtained by FISH analysis with specific probes for PAOs support the presumption that not only one specific organism is responsible for the EBPR. In our case Tetrasphaera spp. dominated the PAO community, but other possible PAOs, like Microlunatus spp. and members of the Rhodocyclus group, were also detected.     

Performance evaluation of an oily industrial wastewater treatment system using the application of activated sludge model No. 3

The purpose of the present study was to adapt the activated sludge model No. 3 (ASM3) to the characteristics of oily industrial wastewater, determining the utmost significant and appropriate kinetic as well as stoichiometric parameters. An oily industrial wastewater treatment system was simulated to assess ASM3 validation and perform sensitivity analysis using the STOAT program. The obtained results revealed that the ASM3 model, which was calibrated after adding the Arrhenius equation into consideration, provided strong correlations with the analytical results of chemical oxygen demand (COD), total suspended solids (TSS), mixed liquor volatile suspended solids (MLVSS) and total suspended solids in the return activated sludge flow (TSS in RAS) concentrations. The values of modelled effluent COD and TSS are very close to those corresponding real values of the treated wastewater by a difference of between 0.5% and 1.5%. Thus, this model becomes successful in representing oily industrial wastewater treatment as a new trend added to the traditional modelling of sewage treatment.     

Wastewater treatment sludge as a raw material for the production of Bacillus thuringiensis based biopesticides

Seven wastewater sludges of different origins and types were used as an alternate culture medium for producing Bacillus thuringiensis variety kurstaki HD-1. The sludge samples were used under three different preparations: without pre-treatment, with acid treatment (hydrolysed sludge) and the supernatant obtained after centrifugation of the hydrolysed sludge. The sludge composition varied widely with origin and the type of sludge. Growth and sporulation were evaluated by the total viable cell count and spore count of the preparations. Growth, sporulation and endotoxin production were affected by the sludge origin. Hydrolysed sludge gave the highest viable cell and spore counts while the liquid phase (supernatant) gave the lowest. Non-hydrolysed primary sludge from Valcartier was unable to sustain bacterial growth because of its low pH. Bioassays were conducted against larvae of spruce budworm to evaluate entomotoxic potential of the preparations obtained. In general, sludge hydrolysis increased the entomotoxicity yields. Similar entomotoxicity was observed in Black Lake secondary sludge (4100 IU/microL) as that obtained in the reference soya medium (3800 IU/microL). The use of the sludge supernatant (liquid phase) was not recommended due to the low entomotoxic potential obtained.     

Preliminary testing of a rapid coupled methodology for quantitation/viability determination of helminth eggs in raw and treated wastewater

The use of raw and treated wastewater for irrigation of crops is a common practice in Mexico. In force since January 1997, the Water Quality Norms for helminth ova refer only to the number of eggs that can be present in the irrigation water, and ignore the assessment of viability because the conventional techniques are not suitable for routine application. However, since viability is of prime importance in the epidemiology of parasitic infections, work was done for the development of a rapid and reliable method to detect a metabolic activity or a physiological characteristic related to viability. As a result, a vital staining procedure was developed and validated, and it was coupled with two rapid quantitative procedures previously assessed, therefore, permitting the determination of the total number of helminth eggs and, simultaneously, the viable and non-viable fraction. The average recovering efficiency of the rapid quantitative methods is 54% for raw wastewater and 77% for treated wastewater; the staining technique for viability is equally reliable (within the 95% confidence limits) as the conventional in vitro procedure; the overall test takes from 4 to6 h to be completed when processing up to four samples at the same time, and the cost per sample (for unrecoverable materials) ranges from US$2.50 to $4.00.     

Degradation of phthalate esters in an activated sludge wastewater treatment plant

Efficient removal of phthalate esters (PE) in wastewater treatment plants (WWTP) is becoming an increasing priority in many countries. In this study, we examined the fate of dimethyl phthalate (DMP), dibutyl phthalate (DBP), butylbenzyl phthalate (BBP), and di-(2-ethylhexyl) phthalate (DEHP) in a full scale activated sludge WWTP with biological removal of nitrogen and phosphorus. The mean concentrations of DMP, DBP, BBP, and DEHP at the WWTP inlet were 1.9, 20.5, 37.9, and 71.9 μg/L, respectively. Less than 0.1%, 42%, 35%, and 96% of DMP, DBP, BBP, and DEHP was associated with suspended solids, respectively. The overall microbial degradation of DMP, DBP, BBP, and DEHP in the WWTP was estimated to be 93%, 91%, 90%, and 81%, respectively. Seven to nine percent of the incoming PE were recovered in the WWTP effluent. Factors affecting microbial degradation of DEHP in activated sludge were studied using [U-¹⁴C-ring] DEHP as tracer. First order rate coefficients for aerobic DEHP degradation were 1.0×10⁻², 1.4×10⁻², and 1.3×10⁻³ at 20, 32, and 43 °C, respectively. Aerobic degradation rates decreased dramatically under aerobic thermophilic conditions (<0.1×10⁻² h⁻¹ at 60 °C). The degradation rate under anoxic denitrifying conditions was 0.3×10⁻² h⁻¹, whereas the rate under alternating conditions (aerobic-anoxic) was 0.8×10⁻² h⁻¹. Aerobic DEHP degradation in activated sludge samples was stimulated 5-9 times by addition of a phthalate degrading bacterium. The phthalate degrading bacterium was isolated from activated sludge, and maintained a capacity for DEHP degradation while growing on vegetable oil. Collectively, the results of the study identified several controls of microbial PE degradation in activated sludge. These controls may be considered to enhance PE degradation in activated sludge WWTP with biological removal of nitrogen and phosphorus.