Cake creep during filtration of flocculated manure

Animal production is becoming increasingly concentrated on large farms. Large amounts of manure are produced of which the N:P ratio often differs from the crop requirements. A possible solution is to separate manure into a P-rich solid fraction and a liquid fraction, which would improve the distribution of N and P on the fields. Filtration is a useful method for such a separation. Furthermore, chemicals can be added to flocculate the solids and thereby increase the filterability i.e. the specific filter-cake resistance can be reduced from 10¹⁵ m/kg to 10¹¹ m/kg. Both the amount of added chemicals, and the mixing procedure affect the result, and lab-scale experiments are often used to study how these pre-treatments influence the filtration process. However, the existing mathematical filtration models are based on filtration of inorganic particles and cannot simulate the filtration data obtained when manure is filtered. Hence, it is not possible to scale up the experiments, and it is therefore difficult to optimize the flocculation and estimate the needed filter media area. Similar problems have been observed when sewage sludge and synthetic core-shell colloids are filtered, and it has been suggested that the discrepancy between the filtration theory and the observed filtration behaviour is due to a time-dependent collapse of the formed cake (creep). This can also explain the observed behaviour when flocculated manure is filtered. The filtration data can be simulated if cake creep is adopted in the filtration model. The calculation shows that the specific filter-cake resistance increases by a factor of 3 during the filtration. Thus, the impact of cake creep is significant when organic materials such as manure are filtered.