Modelling and evaluating municipal solid waste management strategies in a mega-city: the case of Ho Chi Minh City.

Ho Chi Minh City is a large city that will become a mega-city in the near future. The city struggles with a rapidly increasing flow of municipal solid waste and a foreseeable scarcity of land to continue landfilling, the main treatment of municipal solid waste up to now. Therefore, additional municipal solid waste treatment technologies are needed. The objective of this article is to support decision-making towards more sustainable and cost-effective municipal solid waste strategies in developing countries, in particular Vietnam. A quantitative decision support model is developed to optimise the distribution of municipal solid waste from population areas to treatment plants, the treatment technologies and their capacities for the near future given available infrastructure and cost factors.

Effect of low dosages of powdered activated carbon on membrane bioreactor performance.

Previous research has demonstrated that powdered activated carbon (PAC), when applied at very low dosages and long SRTs, reduces membrane fouling in membrane bioreactors (MBRs). This effect was related to the formation of stronger sludge flocs, which are less sensitive to shear. In this contribution the long-term effect of PAC addition was studied by running two parallel MBRs on sewage. To one of these, PAC was dosed and a lower fouling tendency of the sludge was verified, with a 70% longer sustainable filtration time. Low PAC dosages showed additional advantages with regard to oxygen transfer and dewaterability, which may provide savings on operational costs.

Why low powdered activated carbon addition reduces membrane fouling in MBRs.

Previous research had demonstrated that powdered activated carbon (PAC), when applied at very low dosages and long SRTs, reduces membrane fouling in membrane bioreactor (MBRs). In this contribution several mechanisms to explain this beneficial effect of PAC were investigated, including enhanced scouring of the membrane surface by PAC particles, adsorption of membrane foulants by PAC and subsequent biodegradation and a positive effect of PAC on the strength of the sludge flocs. It was concluded that the latter mechanism best explains why low dosages of PAC significantly reduce membrane fouling. Cheaper alternatives for PAC may have a similar effect.

Low dose powdered activated carbon addition at high sludge retention times to reduce fouling in membrane bioreactors.

The addition of a low concentration of PAC (0.5gL(-1) of sludge, i.e. a dose of 4mgL(-1) of wastewater), in combination with a relatively long SRT (50 days), to improve membrane filtration performance was investigated in two pilot-scale MBRs treating real municipal wastewater. Continuous filterability tests at high flux showed the possibility to run for 18h at 72Lm(-2)h(-1) and 180h at 50Lm(-2)h(-1), while significant fouling occurred without PAC. In addition, measurements of the critical flux showed an increase of 10% for this strategy. Low dosage and high retention time makes it feasible and cost effective. Further advantages with regard to permeate quality and possible micropollutants removal are currently under investigation.

Biochemical ripening of dredged sediments. Part 1. Kinetics of biological organic matter mineralization and chemical sulfur oxidation.

After dredged sediments have settled in a temporary upland disposal site, ripening starts, which turns waterlogged sediment into aerated soil. Aerobic biological mineralization of organic matter (OM) and chemical oxidation of reduced sulfur compounds are the major biochemical ripening processes. Quantitative data describing these processes are scarce. Therefore, aerobic oxidation and mineralization of five previously anaerobic dredged sediments were studied during a 160-d laboratory incubation experiment at 30 degrees C. A double exponential decay model could adequately describe sulfur oxidation and OM mineralization kinetics. During the first 7 d of incubation, 23 to 80% of the total sulfur was oxidized, after which no further sulfur oxidation was observed. Oxygen used for sulfur oxidation amounted up to 95% of the total oxygen uptake in the first 7 d and up to 45% of the oxygen uptake during the entire incubation period. Mineralization rates of the rapidly mineralizable OM fractions that degraded during the first 14 to 28 d of incubation were 10(2) to 10(3) times higher than the mineralization rates of the slowly mineralizable OM during the remaining period. First-order mineralization rates of the slowly mineralizable OM were 0.22 x 10(-3) to 0.54 x 10(-3) d(-1) and can be compared with those of terrestrial soils. Yields of biomass on substrate ranged from 0.08 to 0.45 g C(biomass)/g C(OM) and appeared to be higher for rapidly mineralizing OM than for slowly mineralizing OM. The results of this study can be used to optimize conditions during temporary disposal of sediments, to estimate the potential decrease in OM, and for future studies on the possible link between OM mineralization and degradation of hydrophobic organic contaminants.

Biochemical ripening of dredged sediments. Part 2. Degradation of polycyclic aromatic hydrocarbons and total petroleum hydrocarbons in slurried and consolidated sediments.

Ripening of polycyclic aromatic hydrocarbons (PAH) and total petroleum hydrocarbons (TPH) polluted dredged sediment can be considered as a bioremediation technique. Aerobic biodegradation of PAH and TPH was studied in five previously anaerobic-slurried sediments during a 350-d laboratory incubation experiment. In addition, oxygen penetration and degradation of PAH and TPH were studied in three consolidated (physically ripened) sediments. All experiments were conducted in the laboratory at 30 degrees C. A double exponential decay model could adequately describe PAH and TPH degradation kinetics in the slurried sediments. First-order degradation rate constants for the rapidly degradable fractions (12-58%) were approximately 0.13 and 0.058 d(-1) for PAH and TPH, respectively, whereas the rate constants for the slowly degradable fractions were approximately 0.36 x 10(-3) (PAH) and 0.66 x 10(-3) d(-1) (TPH). Rate constants for the rapidly and slowly degrading fractions have the same order of magnitude as the mineralization rate constants of the rapidly and slowly mineralizing organic matter (OM) fractions in the sediments. Oxygen uptake by degradation of PAH and TPH was negligible compared to the oxygen uptake by sulfur oxidation and OM mineralization. In consolidated sediments, PAH and TPH degradation was limited to the oxygenated part. Amounts of PAH and TPH that degraded in the oxygenated parts of the consolidated sediments during 21 d of incubation were similar to the amounts that degraded during 21 d in the slurried sediments.

Amorphous and condensed organic matter domains: the effect of persulfate oxidation on the composition of soil/sediment organic matter.

The composition of amorphous and condensed soil/sediment organic matter (SOM) domains was investigated for one soil sample and four sediment samples. These samples were oxidized with persulfate to remove amorphous SOM, before and after which the composition of SOM was studied by thermogravimetric analysis, pyrolysis-GC/MS, and cross polarization magic angle spinning 13C-NMR. Comparison of the SOM composition before and after oxidation showed that condensed SOM was more thermostable and less polar than amorphous SOM. Condensed SOM was relatively low in O-alkyl C and carboxyl C and it was likely to contain only small amounts of labile organic components (carbohydrates, peptides, fatty acids). Apart from these general characteristics, the composition of the condensed and amorphous domains appeared to be highly dependent on the origin and nature of the SOM investigated. Condensed domains in relatively undecomposed SOM were enriched in aliphatic C, whereas condensed domains in relatively weathered SOM were enriched in aromatic C. Altogether, the compositional changes upon persulfate oxidation were similar to the compositional changes upon humification, which supports the idea that weathered SOM is more condensed than the original material.

The estimation of PAH bioavailability in contaminated sediments using hydroxypropyl-beta-cyclodextrin and Triton X-100 extraction techniques.

A study was conducted to investigate whether cyclodextrins and surfactants can be used to predict polycyclic aromatic hydrocarbon (PAH) bioavailability in contaminated sediments. Two sediment samples were extracted with aqueous solutions of hydroxypropyl-beta-cyclodextrin (HPCD) and Triton X-100. PAH removal during extraction was compared with PAH removal during biodegradation and solid-phase extraction. The latter two methods were used as reference methods to establish which part of the PAHs could be biodegraded and to what extent biodegradation was governed by bioavailability limitations. It was demonstrated that HPCD extraction followed solid-phase extraction and removed primarily readily bioavailable PAHs, while Triton X-100 extracted both readily and poorly bioavailable PAHs. Moreover, HPCD did not affect the degradation of PAHs in biodegradation experiments, while Triton X-100 enhanced the degradation of low molecular weight PAHs. It was concluded that HPCD extraction may provide a good method for the prediction of PAH bioavailability. Triton X-100 extraction is unfit for the prediction of PAH bioavailability.