Characterisation of source-separated household waste intended for composting

Abstract

Large-scale composting of source-separated household waste has expanded in recent years in the Nordic countries. One problem can be low pH at the start of the process. Incoming biowaste at four composting plants was characterised chemically, physically and microbiologically. The pH of food waste ranged from 4.7 to 6.1 and organic acid concentration from 24 to 81 mmol kg(-1). The bacterial diversity in the waste samples was high, with all samples dominated by Gammaproteobacteria, particularly Pseudomonas and Enterobacteria (Escherichia coli, Klebsiella, Enterobacter). Lactic acid bacteria were also numerically important and are known to negatively affect the composting process because the lactic acid they produce lowers the pH, inhibiting other bacteria. The bacterial groups needed for efficient composting, i.e. Bacillales and Actinobacteria, were present in appreciable amounts. The results indicated that start-up problems in the composting process can be prevented by recycling bulk material and compost.

Fig. 1

Organic acids detected in the waste samples. Error bars indicate the standard deviation of the total acid concentration. Triplicate samples, except D1a (duplicate) and B1, D1b, D1c (single values).

Fig. 2

Bacterial diversity in waste samples determined by sequencing of cloned 16S rRNA genes. Sequenced clones with a similarity >94% are included. Only four major classes are included. ‘Other groups’ denotes bacterial sequences with similarity to classes other than the four major bacterial classes identified here. ‘Unidentified’ denotes bacterial sequences with no close similarity to sequences in the nucleotide database. The identity of the samples is given in Table 1. Replicate samples are indicated by Roman numerals.

Fig. 3

Visualisation of microarray results for all wastes. The probe names are listed in each row, while columns represent the different samples. The SNR values for all probes were normalised to the UNIV 1389 probe, and attributed a value between 0 and 1 (UNIV 1389 = 1). The relative intensity of the normalised SNR signal of probes is indicated by the depth of shading. Only probes which had a maximum signal for all hybridisations above the threshold value of 2 were included in the table.

Fig. 4

Loading plot obtained by redundancy analysis, depicting the organisms responsible for community differences between the wastes. The two axes represent 56.1% of the explained variance. The vectors show the covariance structure of the probe signals.

Fig. 5

Gas-filled pore volume as a function of bulk density for waste mixtures at the three plants A–C.