Real Waste Streams Research Articles

An analysis of the composition and metal contamination of plastics from waste electrical and electronic equipment (WEEE)

The compositions of three WEEE plastic batches of different origin were investigated using infrared spectroscopy, and the metal content was determined with inductively coupled plasma. The composition analysis of the plastics was based mainly on 14 samples collected from a real waste stream, and showed that the major constituents were high impact polystyrene (42wt%), acrylonitrile–butadiene–styrene copolymer (38wt%) and polypropylene (10wt%). Their respective standard deviations were 21.4%, 16.5% and 60.7%, indicating a considerable variation even within a single batch. The level of metal particle contamination was found to be low in all samples, whereas wood contamination and rubber contamination were found to be about 1wt% each in most samples. In the metal content analysis, iron was detected at levels up to 700ppm in the recyclable waste plastics fraction, which is of concern due to its potential to catalyse redox reactions during melt processing and thus accelerate the degradation of plastics during recycling. Toxic metals were found only at very low concentrations, with the exception of lead and cadmium which could be detected at 200ppm and 70ppm levels, respectively, but these values are below the current threshold limits of 1000ppm and 100ppm set by the Restriction of Hazardous Substances directive.

Biochemical mediator demand--a novel rapid alternative for measuring biochemical oxygen demand.

The biochemical oxygen demand (BOD) test (BOD5) is a crucial environmental index for monitoring organic pollutants in waste water but is limited by the 5-day requirement for completing the test. We have optimised a rapid microbial technique for measuring the BOD of a standard BOD5 substrate (150 mg glucose/l, 150 mg glutamic acid/l) by quantifying an equivalent biochemical mediator demand in the absence of oxygen. Elevated concentrations of Escherichia coli were incubated with an excess of redox mediator, potassium hexacyanoferrate(III), and a known substrate for 1 h at 37 degrees C without oxygen. The addition of substrate increased the respiratory activity of the microorganisms and the accumulation of reduced mediator; the mediator was subsequently re-oxidised at a working electrode generating a current quantifiable by a coulometric transducer. Catabolic conversion efficiencies exceeding 75% were observed for the oxidation of the standard substrate. The inclusion of a mediator allowed a higher co-substrate concentration compared to oxygen and substantially reduced the incubation time from 5 days to 1 h. The technique replicates the traditional BOD5 method, except that a mediator is substituted for oxygen, and we aim to apply the principle to measure the BOD of real waste streams in future work.

Simulated recycling of post-consumer high density polyethylene material

Post-consumer high density polyethylene (HDPE) was examined by a test procedure consisting of repeated cycles of alternating extrusion and subsequent accelerated thermo-oxidative ageing. The material studied was collected from a real waste stream of HDPE bottles. The effect of addition of a commercial re-stabiliser was also studied. The material was evaluated in terms of mechanical and chemical properties, such as elongation at break, tensile strength, melt flow rate, oxygen induction temperature and molar mass distributions. The results show that after simulated recycling the HDPE material still has considerable resistance against thermal degradation. Upon an additional period of accelerated ageing the stability of the material was still high. The presence of re-stabiliser, introduced during extrusion in the simulated recycling procedure, increased the resistance to subsequent thermo-oxidative ageing. However, significant crosslinking and/or molecular enlargement were found.