Fouling mitigation in membrane distillation processes during ammonia stripping from pig manure

Abstract

Over time fouling leads to membrane wetting. This is the biggest obstacle to widespread use of membrane distillation (MD) for ammonia removal from animal slurry. Feed pretreatment and cleaning strategies of membrane surfaces are the most common methods to prevent or diminish fouling phenomena. This study investigates preliminary fouling of polypropylene (PP), and polytetrafluoroethylene (PTFE) membranes. A model manure solution was used as feed. In addition, cleaning efficiencies with deionized water, NaOH/citric acid, and Novadan agents were studied. Further microfiltration and ultrafiltration were examined as manure pretreatment to diminish fouling. To this end polyvinylidene fluoride membranes (PVDF 0.2 µm and 150 kDa respectively) were used. Organic fouling was shown to be dominant. For the model manure solution, the fouling comprised lipids, carbohydrates and proteins. For pig slurry the fouling additionally contained carboxylates, free fatty acids and lignin. Among the tested cleaning strategies, Novadan agents were the most successful in removing proteins and carbohydrates from the PTFE membrane while it only removed proteins from the PP membrane. Using microfiltration or ultrafiltration as a pretreatment prior to MD doubled the ammonia mass transfer coefficient for the PTFE membrane, while for the PP membrane, the ammonia mass transfer coefficient was increased 4-fold.

Fouling in Membrane Distillation, Osmotic Distillation and Osmotic Membrane Distillation

“…This technique is commonly used for characterization of MD membranes. As an example, Zarebska, et al [103] used AFM morphological analysis to understand the observed decrease in contact angle and surface charge of PTFE and PP membranes used in MD. The three-dimensional AFM pictures of virgin and fouled PTFE and PP membranes with model manure solution and pig manure after sieving with MF and UF showed that the foulants accumulate in the "valleys" leading to a drop of surface roughness [103].…”

“…It is a measure of the magnitude of electrostatic repulsion or attraction between particles. In general, zeta potential is a key indicator of the stability of colloidal particles and it is a good method to describe double-layer properties of colloidal dispersions [15,103]. Various factors affect the zeta potential such as the concentration, composition, temperature and pH of the solution as well as the membrane surface properties (i.e., charge, roughness, chemical heterogeneity, etc.)…”

“…Zarebska, et al [103] measured the streaming zeta potential of PP and PTFE membranes after their use in MD for the treatment of different pretreated model manure feed solutions. It was stated that new PP and PTFE exhibited a small negative zeta potential (above −30 mV, at pH ≈ 9).…”

“…In this technique the sample does not need any specific type of preparation. Further details about this technique can be found in [103,129].…”

“…It was proved that the fouling layer was formed by only organic compounds, whereas no indication of inorganic scaling was detected [129]. Zarebska, et al [103] used ATR-FTIR to detect functional groups in PP and PTFE membranes used in MD for the treatment of model manure solution. This technique showed that fouling layer composition was not uniform across the entire membrane surface and all the bands found by ATR-FTIR spectroscopy were assigned to organic fouling.…”