## 1 Introduction  
Cheese whey CW is a highly concentrated liquid waste generated during the production of curd, kefir and some types of cheese from milk. CW is characterized by a high content of organic substances, ranging from 25 to 80 g COD (chemical oxygen demand)/L. Annual milk production in Russia is more than 30 million tons, while the specific production of CW is estimated at 0.8-0.9 L per L processed milk [1]. Due to its high content of lactose (45-50 g/L), proteins (6-8 g/L), lipids (4-5 g/L), as well as macro-and microbeements, water and fat-soluble vitamins, CW can be used as an inexpensive raw material for the production of biofuels, in particular biohydrogen, by its dark fermentation (DF) [1,2]. Theoretically, eight moles of hydrogen can be obtained from one mole of lactose. However, the actual yield depends on many factors and in practice rarely exceeds four mol H\({}_{2}\)/mol lactose [3]. The choice of inoculum is essential for the efficiency of the DF process because hydrogen producers can form various metabolites that affect the specific yield of hydrogen. The microbial community of the inoculum converts lactose into soluble metabolites (alcohols, acids), CO\({}_{2}\) and H\({}_{2}\), according to the following general equation:  
\[\text{Lactose}\rightarrow\text{glucose}+\text{galactose}\rightarrow\text{soluble metabolites}+\text{CO}_{2}+\text{H}_{2} \tag{1}\]  
Galactose obtained in Equation (1) is decomposed further in the chain of reactions to form glucose 6-phosphate via the Leloir pathway [4]. Glucose decomposes with the formation of H\({}_{2}\), CO\({}_{2}\) and other products in the chain of biochemical reactions described in detail in several works [5,6], so if the final product is acetic acid, the biohydrogen yield is at a maximum 4 mol H\({}_{2}\)/mol glucose. At the same time, when butyric acid is formed, theoretically, only 2 mol H\({}_{2}\)/mol of glucose can be released. A decrease in hydrogen production is often due to its consumption by methanogens presented in the anaerobic sludge, therefore, for DF, the stage of preliminary selective enrichment of hydrogen-forming microorganisms and the elimination of hydrogenotrophic methanogens in the inoculum is important. Among the various pretreatment methods, thermal inactivation has become the most widespread due to its relative simplicity [7]. Other methods should also be noted: treatment with acid [8], alkali [9], antimethanogenic compounds or substances addition [10] or a technological method--increasing the organic loading rate (OLR) [11]. Feeding mode and the bioreactor configuration are significant factors affecting the availability of the substrate for hydrogen producers, the rate of biomass dilution, mixing conditions and mass transfer processes. Depending on the means of feeding, the production of biohydrogen can be carried out in a batch, semi-continuous and continuous modes. The batch mode is relatively simple in technological design and the control of the DF process. Investigations of the DF of CW in a batch mode were mainly aimed at comparing temperature conditions [12], initial substrate concentrations [13], and H\({}_{2}\) producers [14]. A semi-continuous mode is recommended for the processing of viscous and solid wastes such as food waste and lignocellulosic biomass, where physical contact between the substrate and microorganisms is limited [15]. This mode of reactor operation provides a better hydrolysis rate, prevents pipe clogging, and promotes effective retention of biomass in the reactor. Research in the continuous production of biohydrogen from CW is mainly aimed at optimizing the DF process using various reactor configurations such as membrane bioreactor (MBR) [16], continuous stirred tank reactor (CSTR) [17], up-flow anaerobic sludge bed reactor (UASB) [18], anaerobic packed-bed (APB) [19] and anaerobic fluidized bed reactor (AFBR) [20,21]. Continuous reactors are usually started in batch mode to ensure proper inoculum preparation and pre-treatment.