Sludge Streams Research Articles

Assesing the Potential for Struvite Recovery at Sewage Treatment Works

Struvite in wastewater treatment plants was identified as early as 1939. Problems with struvite formation date back to the 1960s when it was noticed at the Hyperion treatment plant, Los Angeles. Operators at the plant noticed crystalline deposits on the underside of post digestion screens. The digested sludge stream was diluted and it was thought the problem was solved, until five years later when the normal gravity flow of digested sludge had decreased to such a stage that pumping was required. This paper reports the findings of a series of experiments undertaken to identify the potential of recovering struvite from sludge liquors. Seven sludge treatment works (STW) have been investigated including one detail. A number of the works has the potential to form over 100 mg l−1 of struvite.

Struvite Precipitation in the Sludge Stream at Slough Wastewater Treatment Plant and Opportunities for Phosphorus Recovery

Struvite precipitation has been a problem at Slough wastewater treatment works for a number of years. The plant has recently been extended with a 3-stage Bardenpho plant which has reduced the phosphorus concentration in the effluent and led to an increase in struvite precipitation in the digested sludge pipelines. These problems have been overcome in the short term by dissolving the struvite in 10% sulphuric acid. It is hoped to provide longer term protection against struvite precipitation by installing magnetic water softening devices at key locations. This form of protection is already providing some benefit at one point on the site. The phosphorus concentration and loads at a number of points on the site have been investigated. By combining two streams, the centrate from the sludge dewatering plant and the liquor from thickening the waste activated sludge, a relatively clean stream containing 160 mgl−1 of soluble phosphate, a load of 136 kgd−1 can be produced. In order to make best use of this stream it is important to prevent struvite formation at other points in the plant where the precipitation can cause operational problems.

Pilot plant study to assess the fate of two volatile methyl siloxane compounds during municipal wastewater treatment

AbstractThe environmental fate ofoctamethylcyclotetrasiloxane(D4) and decamethylcyclopentasiloxane (D5) during municipal wastewater treatment has been investigated in an activated sludge pilot plant by continuously dosing the influent with exagerated amounts of the compounds and measuring their emissions in the off‐gas, effluent, and sludge streams. The total removal was found to be 86.4 ± 3.9% for D4 and 95.8 ± 1.5% for D5 (±1 SD statistical uncertainty). However, low overall mass balances were found for both compounds in the pilot system. In order to elucidate the detailed removal mechanisms for these two compounds, the model TOXCHEM was used to simulate their removal and analyze their mass balances in the pilot system. Experimental and model results indicated that the low mass balances were caused by the significant underestimation of the primary sludge removal by the grab sampling method used for sludge analyses. This underestimation may be due to the high variability of the primary influent suspended solids concentration and hence inhomogeneity in the primary sludge underflow. When this underestimation was taken into account, primary sludge removal and volatilization in the aeration basin were found to contribute approximately equally to the removal of D4 and D5 in the pilot plant system. The present results are useful for the environmental risk assessment of these two siloxane compounds in the unlikely event of their point source discharge into municipal wastewater systems.

Effect of mixing on the rheological characteristics of conditioned sludge: full-scale studies

Conditioning of wastewater sludges with polyelectrolytes prior to dewatering is necessary to increase the sludges' dewaterability. During this conditioning process adequate mixing of the conditioner and the sludge must be attained. This research reports on results from full-scale experiments conducted to examine the effect of mixing parameters upon the conditioned sludge and liquid stream rheological characteristics. Full-scale experiments were conducted at the 90 MGD Wilmington Pollution Control Facility. Varied mixing conditions were the rotational speed of the flocculation tank prior to dewatering and the filtrate recycling to the flocculation tank. The rheological changes in the conditioned sludge were detected using an on-line ZenoFloc™ system. Although previous investigations have shown effects of such variations with regard to CST, use of a rheometer as indication of the rheological changes also showed that G variations led to different rheological levels. This allows some insight into conditioning mechanisms as well as optimization. Results also showed that mixing must be optimized and controlled for better dewatering performance.

Effect of mixing on the rheological characteristics of conditioned sludge: Full-scale studies

Conditioning of wastewater sludges with polyelectrolytes prior to dewatering is necessary to increase the sludges' dewaterability. During this conditioning process adequate mixing of the conditioner and the sludge must be attained. This research reports on results from full-scale experiments conducted to examine the effect of mixing parameters upon the conditioned sludge and liquid stream rheological characteristics.Full-scale experiments were conducted at the 90 MGD Wilmington Pollution Control Facility. Varied mixing conditions were the rotational speed of the flocculation tank prior to dewatering and the filtrate recycling to the flocculation tank. The theological changes in the conditioned sludge were detected using an on-line ZenoFlocTm system.Although previous investigations have shown effects of such variations with regard to CST, use of a rheometer as indication of the rheological changes also showed that G variations led to different rheological levels. This allows some insight into conditioning mechanisms as well as optimization. Results also showed that mixing must be optimized and controlled for better dewatering performance.

Dynamic mathematical modelling of sequencing batch reactors with aerated and mixed filling period

Sequencing batch reactors (SBRs) can be successfully operated for both carbon and nutrient removal, including nitrogen and phosphorus. The major elements of design that accomplish population dynamics control to prevent filamentous sludge bulking, cycle time, oxygen supply, biological nitrification, denitrification, phosphorus removal and solids-liquid separation need to be set in such a way that sufficiently optimal conditions are provided to permit the reactions and processes to take place. SBR processing using cyclic activated sludge technology employs biological selectors in the inlet part of the SBR system and a minor sludge recycle stream to ensure influent wastewater is mixed with activated sludge flocs to create favourable conditions for kinetic and metabolic selection of microorganisms producing floccules. Reaction volume, in addition to the designated bottom water level volume, is variable through time fed-batch reactor mode of operation. A mathematical model that describes volume changes and simultaneously the biodegradation kinetics has been developed. The model describes theoretical behaviour of selected parameters of volume, suspended solids concentration, OUR, ammonia and nitrate nitrogen in the selector compartment and the main aerated basin in ideally mixed and filled reactors of the cyclic system during the phase of mixed-fill (selector) and aerated and non-aerated fill (main aeration reactor basin).

Dynamic mathematical modelling of sequencing batch reactors with aerated and mixed filling period

Sequencing batch reactors (SBRs) can be successfully operated for both carbon and nutrient removal, including nitrogen and phosphorus. The major elements of design that accomplish population dynamics control to prevent filamentous sludge bulking, cycle time, oxygen supply, biological nitrification, denitrification, phosphorus removal and solids-liquid separation need to be set in such a way that sufficiently optimal conditions are provided to permit the reactions and processes to take place. SBR processing using cyclic activated sludge technology employs biological selectors in the inlet part of the SBR system and a minor sludge recycle stream to ensure influent wastewater is mixed with activated sludge flocs to create favourable conditions for kinetic and metabolic selection of microorganisms producing floccules. Reaction volume, in addition to the designated bottom water level volume, is variable through time fed-batch reactor mode of operation. A mathematical model that describes volume changes and simultaneously the biodegradation kinetics has been developed. The model describes theoretical behaviour of selected parameters of volume, suspended solids concentration, OUR, ammonia and nitrate nitrogen in the selector compartment and the main aerated basin in ideally mixed and filled reactors of the cyclic system during the phase of mixed-fill (selector) and aerated and non-aerated fill (main aeration reactor basin).

Nitrifying bacteria in the influent to a wastewater treatment plant - influence and importance on nitrifying capacity

Cells of Nitrobacter sp. were enumerated in the influent to Bromma wastewater treatment plant using monoclonal antibodies in a competitive ELISA. By comparing this method with the most probable number technique we conclude that the immunological method is much more relevant for determining Nitrobacter sp. The amount of Nitrobacter sp. cells in the influent was measured once every second month during one year. Examinations concerning variations during one week and one day respectively were conducted. Finally a mass balance was constructed describing the amount of nitrifiers at different sites in the plant. The analysis showed that the amount of Nitrobacter sp. cells varies during the year but no obvious pattern could be observed. At the middle of an average week higher amounts were recorded. The major part of the nitrifiers was found in the biological unit and most of them have entered the wastewater treatment plant by the influent. Only small quantities were found in water from secondary clarifiers, filters and different sludge streams. 90% of the incoming cells of Nitrobacter sp. in the influent were captured in the plant. 5% of the bacteria were lost with the effluent and another 5% with the digested dry solids.

Nitrifying bacteria in the influent to a wastewater treatment plant - influence and importance on nitrifying capacity

Cells of Nitrobacter sp. were enumerated in the influent to Bromma wastewater treatment plant using monoclonal antibodies in a competitive ELISA. By comparing this method with the most probable number technique we conclude that the immunological method is much more relevant for determining Nitrobacter sp. The amount of Nitrobacter sp. cells in the influent was measured once every second month during one year. Examinations concerning variations during one week and one day respectively were conducted. Finally a mass balance was constructed describing the amount of nitrifiers at different sites in the plant. The analysis showed that the amount of Nitrobacter sp. cells varies during the year but no obvious pattern could be observed. At the middle of an average week higher amounts were recorded. The major part of the nitrifiers was found in the biological unit and most of them have entered the wastewater treatment plant by the influent. Only small quantities were found in water from secondary clarifiers, filters and different sludge streams. 90% of the incoming cells of Nitrobacter sp. in the influent were captured in the plant. 5% of the bacteria were lost with the effluent and another 5% with the digested dry solids.

Fate of polynuclear aromatic hydrocarbons during anaerobic digestion of municipal wastewater sludges

The fate of six polynuclear aromatic hydrocarbons during anaerobic digestion of municipal sludges was investigated in a two-stage pilot-scale digester system. Primary digester removal efficiencies ranged from 23% for acenaphthene to 62% for acenaphthylene ; secondary digester removal efficiencies ranged from 22% for acenaphthylene to 42% for phenanthrene. Most of the remaining mass of candidate compounds was present in the secondary digester waste sludge stream. Less than 2.5% of the candidate compounds entering the digestion system was present in the secondary digester supernatant. A dynamic model of the primary digester incorporating mixed first-order biodegradation kinetics was calibrated for each of the candidate compounds. The biodegradation rate coefficients that were estimated for the primary digester ranged from 0.004 L/g.d for acenaphthene to 0.16 L/g.d for acenaphthylene. The biodegradation rate coefficient for acenaphthene did not significantly differ from zero.

Enhanced nutrient removal by oxidation-reduction potential (ORP) controlled aeration in a laboratory scale extended aeration treatment system

Two laboratory scale extended aeration treatment systems (EATS) were built to study their nutrient removal performance before and after the introduction of oxidation-reduction potential (ORP) controlled aeration. Effects of various ORP set points (40, 70, 110 and 180 mV) and zoning of the aeration tank (AT) were examined. Each system had a 901 aeration tank and the operating HRT and SRT were 20 h and 20 days, respectively. Synthetic wastewater was used and the F/M ratios ranged from 0.13 to 0.15 gCOD/gMLSS/d. All experimental runs including the control yielded a similar COD removal efficiency of 96.4 ± 0.4% and an effluent COD of 14.4 ± 1.3 mg. This indicates that ORP controlled aeration had no significant impact on the carbonaceous removal of the EATS. Very good nitrogen (N) and phosphorus (P) removal was obtained when there was a small anoxic selector (3.8 l) at the head of the AT and the tank zoned into a main ORP zone (72 l) followed by an aerobic zone (18 l). The selector was found very useful in the prevention and curing of bulking as well as in the removal of P. It effectively denitrified the nitrate from the return sludge stream and caused significant P release, that is important to subsequent P uptake in the ORP and aerobic zones. An optimum ORP set point of 110 mV was found. The aeration pattern was characterized by having an equal time length of aeration and non-aeration. The achievable total nitrogen (TN) removal was 93% with a residual TN of only 2.94 mg/l (N). Excellent P removal of 99.5 ± 0.1% was attained under an ORP set point range of 70–180 mV. P was reduced to 0.05 ± 0.01 mg/l (P) in the effluent.

Heavy metal variability of different municipal sludges as measured by atomic absorption and inductively coupled plasma emission spectroscopy

Abstract Six sludge streams at two sewage treatment plants were monitored for three months to determine variability, degree of magnification, and generation rates of heavy metals and nutrients. Measurements of As, Ba, B, Cd, Cr, Cu, Pb, Hg, Mo, Ni, Se, Ag, Zn, K, N, P, ammonia, total solids, and volatile solids showed that most variability was related to the type of sludge (primary sludge, waste activated sludge, digested sludge, dewatered sludge), and to a lesser extent, to the day‐to‐day changes of a particular sludge stream.

The Application of Cross‐flow Microfiltration Technology to the Concentration of Sewage Works Sludge Streams

SYNOPSISA LOCALLY DEVELOPED cross‐flow microfiltration process using woven fabric tubes was used to concentrate waste activated sludge and anaerobic digested sludge from a conventional sewage works.Results showed that waste activated sludge could be concentrated from 5 to 50 g/l total solids. The permeate quality was good (0 to 50 mg/l suspended solids), but deteriorated both with time and increasing feed solids concentration.A cross‐flow microfilter was coupled to a pilot scale anaerobic digester and the digester solids were increased from 26 to 55 g/l total solids. The organic loading to the digester was increased from 1.8 to 3.1 kg volatile solids per m3 per day. The solids retention time was held constant at 26 days, while the liquid retention time was decreased from 26 to 14 days. The permeate quality was significantly better than the supernatant liquor from a comparable digester (suspended solids 122 and 570 mg/l, respectively).Data obtained from the cross‐flow microfiltration of waste activated sludge was used to regress for the constants in a mathematical model of steady‐state flux.

Influence of atomization quality on the destruction of hazardous waste compounds

The correlation between atomization quality and the destruction efficiency of hazardous organic compounds was studied in a turbulent spray flame. The atomization quality was varied by both changing spray nozzle parameters and by inducing disruptive droplet combustion (secondary atomization) within the flame. The primary atomization quality was characterized by laser diagnotic size distribution measurements. The secondary atomization quality was determined from observations of disruptive atomization intensity on a train of monodisperse droplets within a high-temperature laminar reactor. For the primary atomization work, No. 2 fuel oil was doped with four target hazardous organic compounds (acrylonitrile, chloroform, benzene, and monochlorobenzene). The destruction efficiency of these compounds was measured under correct atomizer operating parameters and under off-design conditions in which the spray quality was degraded. The degraded spray quality conditions resulted in decreased destruction efficiency of the waste, and increased combustion intermediate emissions (carbon monoxide and total hydrocarbons). Comparison of measured droplet size distributions with performance showed that destruction efficiency was more closely correlated with the large droplet wing of the dropsize distribution than with the mean droplet size. A droplet evaporation/ trajectory model showed that the appearance of the target compounds in the exhaust corresponded with the fraction of the droplets that passed through the primary reaction zone unevaporated. The distruptive droplet combustion results showed that hazardous waste compounds are capable of inducing secondary atomization. Testing of benzal chloride (which did not cause the turbulent flame reactor showed that the occurrence of disruptive combustion correlated with increased target compound destruction efficiency and reduced combustion intermediate emissions. Thus, the results suggest that the presence of certain compounds or additives in waste streams may assist in obtaining improved performance when primary atomization is poor, as it is for slurry or sludge waste streams.

Treatment in municipal plants: Innovations for removal of phosphorus

The paper provides a background of a variety of different phosphorus removal techniques which have been reported in the literature. Forty-nine different references which may be described under the general headings of “biological”, “chemical”, “chemical-biological”, and “chemical-physical”, have been abstracted and described under a series of salient headings. The manner in which they are described considers the point of application of the process, the percentage removal of phosphorus which was experienced, the cost of the process where this was given, and whether or not the process is related to a concentrated sub-stream within the waste water treatment plant.The main emphasis of the paper deals with the concept of removing phosphorus from waste water by using a sub-stream within the plant which naturally has phosphorus concentrated considerably above the ambient (influent) concentration.A continuous laboratory-scale plant was studied, operating under contact stabilization conditions, and using an anaerobic holding technique on the return activated sludge, phosphorus was removed from the supernatant of the return sludge stream. The plant was operated with and without the anaerobic holding modification, and the results of the operation are reported in the paper. Both before and after the modification, the organic carbon as measured by COD removal was of the order of 90 per cent; and after modification of the plant to release the phosphorus from the return sludge stream, phosphorus removals jumped from 10 to 30 per cent up to a high of 75 to 90 per cent removal. A very slight decrease in sludge activity could be observed due to the anaerobic holding, but the very slight reduction in COD removal was more than offset by the increased removal of soluble phosphorus through the supernatant of the return sludge line.Development of this process onto a full-scale basis is considered as the next stage of this research programme.