The impact of fouling on the process performance of the thermal treatment of pig slurry using tubular heat exchangers.

The aim of this study was to determine the kinetics of fouling and their influence on the performance of a thermal treatment process used for sanitisation of pig slurry. Two temperatures (55 °C and 80 °C) were investigated. One trial was carried out at 55 °C and 80 °C in which the slurry was not re-circulated and one trial at 80 °C in which 100% or 50% of the slurry was re-circulated. Fouling of the heat exchangers was assessed by on-line monitoring of the drop in pressure, changes in treatment temperature, heat transfer coefficients, heat recycling rate, and energy consumption. Similar energy consumption of around 38 kWh m(-3) of effluent was observed at the two temperatures. The operating periods prior to excessive fouling or blockage were 18 days at 55 °C and four days at 80 °C. Recycling treated manure to obtain 50% dilution of the raw feed increased the viable operating period to 14 days at 80 °C but doubled energy consumption. At 55 °C, the significant drop in the target temperature (>7 °C) with fouling severely jeopardised the process. The nature of the decline in performance suggests that the main fouling mechanisms were bio-fouling at 55 °C and organic/mineral deposits at 80 °C. Recycling treated manure enabled the operating period to be extended but increased the total cost of heating. One hundred percent recycling showed that the fouling potential of the manure was largely eliminated after one thermal treatment, suggesting a pretreatment may be advantageous.

Characteristics and composition of fouling caused by pig slurry in a tubular heat exchanger--recommended cleaning systems.

The structure and composition of the fouling deposits caused by pig slurry heated in a tubular heat exchanger were characterized to understand their formation and thus be able to minimize fouling and define effective routine cleaning methods. Two temperatures (55 °C and 80 °C) were investigated. Two types of fouling were identified: organic/mineral and biofilm. The first only formed at temperatures above 50 °C, often during the heating phase, and was the main problem encountered in treatments at 80 °C. Organic/mineral deposits formed a thin compact sub-layer and a thick porous top layer composed of 67-76% minerals, 9-15% proteins, 8-20% carbohydrates and 0-5% fats. Biofilms formed at temperatures between 25 °C and 70 °C in both the cooling and heating sections of the exchanger. This type of fouling predominated at temperatures below 55 °C. The biofilm covered a thin mineral base layer. Strongly acidic or alkaline washing cycle are recommended to clean Type I deposits, while in-line gas-rumbling is recommended for Type II fouling.

Using temperature and time criteria to control the effectiveness of continuous thermal sanitation of piggery effluent in terms of set microbial indicators.

AIM: To determine the minimal conditions (temperature-time), necessary to achieve set sanitation targets for selected microbial indicators during the continuous thermal treatment of pig slurry. METHODS AND RESULTS: The effectiveness of thermal treatment between 55 and 96°C was studied using Escherichia coli, enterococci, sulfite-reducing Clostridia (SRC), mesophilic culturable bacteria (MCB), F+-specific and somatic phages. Identification of SRC and MCB was performed using 16S rRNA gene analysis. Ten minutes at 70°C or 1 h at 60°C was sufficient to reduce the vegetative bacteria by 4-5 log(10), but it had little effect on somatic phages nor on spore formers, dominated by Clostridium sp. At 96°C, somatic phages were still detected, but there was a reduction of 3.1 log(10) for SRC and of 1.4 log(10) for MCB. At 96°C, Clostridium botulinum was identified among the thermotolerant MCB. CONCLUSION: Only those hygienic risks relating to mesophilic vegetative bacteria can be totally eliminated from pig slurry treated at 60°C (60 min) or 70°C (<10 min). SIGNIFICANCE AND IMPACT OF THE STUDY: Hygiene standards based on the removal of the indicators E. coli and enterococci can easily be met by treatment as low as 60°C (enabling, a low-cost treatment using heat recovery). However, even at 96°C, certain pathogens may persist.