Pathogen removal using saturated sand colums supplemented with hydrochar

This PhD study has evaluated hydrochars derived from biowastes as adsorbents for pathogen removal in water treatment. Pathogen removal experiments were conducted by carrying out breakthrough analysis using a simple sand filtration set-up. Glass columns packed by 10 cm sand bed supplemented with minor amount of hydrochar (1.5 %, w/w) were flushed with artificial ground water seeded with test microorganisms at an upward flow rate of 1 mL / min. Either back flushing or deionized water flushing was performed at pathogen retaining columns in order to investigate the pathogen removal mechanism of hydrochar-amended sand columns.

Two home-made two-step reverse transcription-quantitative polymerase chain reaction assays were developed in order to quantify rotavirus in the samples from virus removal experiments. Since the total cost of the assays was mainly determined by the cost of reverse transcriptase, two reverse transcriptases with the lowest consumer price were employed. The efficiencies of home-made assays were comparable to a selected reference commercial kit in analyzing both environmental and laboratory samples, while the total cost of home-made assays was 11 times less. Though home-made assays necessitate more manual operations and time, the low-cost aspect might be appealing in those settings where the expenditure for consumables inhibits laboratories in their functioning.

Hydrochars produced via hydrothermal carbonization of agricultural residue from maize or stabilized sewage sludge were evaluated for adsorptive removal of Escherichia coli. In order to improve the removal capacity, the hydrochars were activated by being suspended in 1M KOH solution (5 g hydrochar / L) for 1 h at room temperature. The activation improved the Escherichia coli removal efficiency from ~70 to ~90 %. In addition, successive detachment experiments carried out by performing back flushing or deionized water flushing into sand columns supplemented with maize-derived hydrochar indicated that the strength of Escherichia coli attachment increased by KOH activation. Also, Escherichia coli removal of sewage sludge-derived hydrochar was evaluated in larger column with 50 cm filter bed for 30 days of intermittent operation. 3 pairs of columns packed with either sand, raw hydrochar or activated hydrochar showed average removal efficiency of 36.5% ± 10.1 (n=60), 24.4% ± 10.5 (n=56) and 91.2% ± 7.5 (n=60), respectively. Idle time of filtration unit did not affect the Escherichia coli removal efficiency of hydrochar-amended columns. The results from material characterization attributed the enhancement in Escherichia coli removal induced from the KOH activation to development of macroporous surface with increased hydrophobicity and surface charge. It was apparent that the activation with the KOH solution removed tar-like substances from hydrochar surface, resulting in exposure of hydrophobic core and development of rough surface structure. Also, deposition of K+ ion in hydrochar was observed, which might have increased the surface charge.

The removal of human pathogenic rotavirus and adenovirus was investigated using hydrochar derived from stabilized sewage sludge or swine faecal waste. Throughout virus removal experiments, rotavirus and adenovirus showed comparable removal. At 1 mL / min flow rate, raw hydrochar (without KOH activation) derived from either feedstock showed mean virus removal efficiency from 2 to 3 > log removal (99 - 100 %). Also flow rates of 2.5 and 5 mL / min were tested using faecal waste-derived hydrochar. The virus removal efficiency remained still high (2.1 log - 3 log) at the elevated flow rates. We speculated that the improvement in virus removal derived from hydrochar supplement is induced by provision of extra hydrophobic surfaces in sand column media. Regardless the type of packing material, successive deionized water flushing into virus-retaining columns released more rotavirus than adenovirus, indicating larger role of the secondary energy minimum in rotavirus retention. It was remarkable, because both types of viruses are similar in their shape and size. This observation provides evidence that virus transport-retention behaviour could be mainly determined by surface characteristics of virus rather than its size and shape. In this sense, the use of model virus needs to be carefully considered when performing water treatment or pathogen transport experiments.

Successful removal of pathogenic virus using faecal waste-derived hydrochar highlights the potential of hydrothermal carbonization technology in less developed regions where modern water-sanitation systems are not affordable. Faecal waste, one of the most important pathogen sources, can be completely sanitized in elevated temperature and pressure during hydrothermal carbonization process. Moreover, the resulting hydrochar can be utilized at water or wastewater treatment. Despite general low-cost aspect of hydrothermal carbonization such as less energy dependency than dry pyrolysis and utilization of waste as a feedstock, the need for high-pressure reactor might hamper the implementation of the technology. Development of localized low-cost reactor, evaluation of hydrochar in its use at agriculture and/or energy production and overall economic analysis are recommended.

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