Recycling ethanol fermentation stillage from food waste could help to resolve the pollution problems, but it would lead to the accumulation of lactic acid (LA), which had a negative effect on ethanol fermentation. In this study, bipolar membrane electrodialysis was used to recover LA as well as other fatty acids in the stillage, and the corresponding parameters that affected this process were investigated. Pretreatment via centrifugation and ultrafiltration reduced the turbidity successfully. Experiments on simulated and real stillage showed that organic acids such as formic acid and acetic acid had a slight effect on the separation of LA, but the existence of chloride ions could form strong anion competition with lactate ions. Simulation experiments with sodium chloride reduced the recovery of LA from 98% to 92%. The recovery rate of LA from real stillage could reach 71.2%. After electrodialysis, the stillage can be reused in ethanol fermentation, the fermentation time was shortened to 24 h, and the ethanol yield increased.

Introduction

For the sustainable development of society, the recycling of waste has become more and more important. Food waste (FW) accounts for a large proportion of municipal solid waste (15%–63%) [1]. It has high organic, volatile solid (85%–95%), and moisture content (75%–85%) and is easily degraded and deteriorated. There are many ways of reusing the waste, such as composting and production of pig feed, biomethane, biodiesel, ethanol, and lactic acid (LA) [2]. The use of FW to produce ethanol can take full advantage of the rich nutrients in FW and obtain fuel. However, a large quantity of stillage will be produced; one way for dealing it is stillage recycle fermentation [3]. This method can reduce pollution and utilize proteins, amino acids, and various metal ions in stillage [4]. In our previous study, LA was determined to be the major accumulated by-product along with stillage recyling. A significant inhibition phenomenon was observed after 5 times recycling. Although the addition of calcium carbonate could alleviate the influence of LA on ethanol fermentation, if we can separate the accumulated LA in the stillage, it will provide better economic benefits. LA is an important natural organic acid. It has received increasing attention as one of the most important building blocks for the production of polylactic acid [5,6]. At present, the main methods of LA production are microbial fermentation and chemical synthesis. Biological fermentation is inexpensive, utilizes a wide range of raw materials, and produces optically pure L- or D-lactic acid, which make it superior to chemical synthesis. More attention has also been paid to the utilization of renewable carbohydrates as a carbon source for LA fermentation [7,8]. However, the major cost of LA production is due to its downstream separation and purification steps [9]. As LA exists in the form of lactate salt in the fermentation broth, acidification and removal of impurities are required for future separation [10]. The extraction methods of LA include precipitation, solution extraction, adsorption, distillation, and nanofiltration [11]. Electrodialysis has the advantages of diversity, sophisticated functions, and technological compatibility [12,13]. Danner et al. used monopolar electrodialysis to extract LA from grass silages and obtained press extraction yields of 31–96 g of lactate per kilogram of silage dry matter (g LA/kg DM) [14]. Chen used conventional electrodialysis (CED) to remove 80% lactate from acid whey [15]. Lopez-Garzon used bipolar membrane electrodialysis (BMED) to recycle LA and other carboxylic acids in the fermentation broth [16].