Feeder Protection, Conductor Sizing, Load Flow and Fault Calculation

Geoff Macangus-Gerrard , in Offshore Electrical Engineering Manual (Second Edition), 2018

Busbar Sizing

Switchboard main busbars must be rated to carry the maximum continuous load which can flow in any healthy power system operating condition. Transient conditions giving rise to higher currents, such as those due to large motors starting or downstream faults, may be tolerated momentarily, provided the protection devices are incorporated which will ensure that the outgoing equipment is removed from the system before the busbars get overheated.

The continuous current rating must be for the busbars as enclosed in their offshore environmental protection, with natural cooling only. This also applies to the switching and isolating devices in the switchboard.

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Process Drives and Starting Requirements

Geoff Macangus-Gerrard , in Offshore Electrical Engineering Manual (Second Edition), 2018

Direct Current Drilling Motors

The conventional arrangement in a drilling rig is to utilise 750   V direct current (DC) machines run from phase angle-controlled thyristor units.

Typically, the machines must be capable of accepting a voltage variation of 0–750 V DC and continuous load currents of 1600  A. For mud pump duty, two motors will run in parallel on one silicon controlled rectifier (SCR) bridge, whilst for the drawworks and rotary table, one SCR bridge will be assigned to each motor with appropriate current limiting devices in operation. The motors must also be capable of producing around 600   kW continuously and 750   kW intermittently at 1100   rpm. These machines are shunt-wound machines with class H insulation, derived from railway locomotive designs. However, as they operate in hazardous areas, the construction is closed air circuit, water cooled to restrict surface temperatures, with the enclosures being pressurised to prevent ingress of explosive gas mixtures.

This type of motor is often provided without a terminal box, the winding tails passing through a sealed and insulated gland to a separate flameproof terminal box. It is recommended, however, that motor-mounted terminal boxes should be used, as the exposed winding tails are difficult to protect mechanically.

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Electrical Systems*

Ken Arnold , Maurice Stewart , in Surface Production Operations: Design of Gas-Handling Systems and Facilities (Second Edition), 1999

Electrical Generating Stations

Where electricity is generated in the facility, generator sizing should consider not only connected electrical loads, but also starting loads and anticipated and non-anticipated expansions. In most installations this is done by developing an electrical load list itemizing the various loads as either continuous, standby or intermittent service. Examples of continuous loads are electric lighting, process pumps and compressors required to handle the design flow conditions, and either quarters heating or air conditioning, whichever is larger. Intermittent loads would include quarters kitchen equipment, washdown pumps, cranes, air compressors and similar devices which are not in use at the same time. The total demand is normally taken as 100% of the continuous loads, 40 to 60% of the intermittent loads and an allowance for future demand. Standby loads do not add to generator demand as they are activated only when another load is out of service.

Generators must be sized to handle the starting current associated with starting the largest motor. On large facilities with many small motors, starting current usually can be neglected unless all the motors are expected to start simultaneously. However, if the total load is dominated by several large motors, the starting load must be considered.

In calculating generator loads it must be remembered that each motor will only draw the load demanded by the process. It is this load and not the nameplate rating of the motor that should be used in the load list. For example, even though a pump is driven by a 100 hp motor, if the process conditions only demand 75 hp, the total load that will be demanded from the generator is 75 hp.

Generators are normally provided with static voltage regulators capable of maintaining 1% voltage regulation from no load to full load. While random ("mush") wound stators are acceptable for smaller units, formed coils are normally preferred for generators of approximately 150 kW or larger. Vacuum-pressure-impregnated (VPI) windings are recommended for all units operating in high-humidity environments.

Smaller generators, typical of those frequently used at production facilities, often cannot provide enough current to operate the instantaneous trip of magnetic circuit breakers used as main circuit breakers under certain conditions. Manufacturers' data should be obtained for units under consideration and, if necessary, a short-circuit boost option or a permanent magnet rotor (PMG) option should be considered. These options will assist the voltage regulator in delivering full exciter voltage and current during periods of severe generator overload and short circuit conditions. This helps assure that the generators are capable of delivering enough current to trip the main circuit breaker.

When generators are specified, it should be realized that both mechanical and electrical requirements differ between units which will be used for standby service and units which will be operated continuously. Typically, standby units have less copper in their windings than continuous duty units, causing standby units to reach higher temperatures if operated continuously, and thus reducing life. Standby units, as classified by most manufacturers, are not to be confused with units which are alternated weekly (or on some other regular basis), but which are operated continuously when they are "on line." This operating mode should be considered "continuous duty."

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Centrifugal Compressors

Jason Wilkes , ... George Talabisco , in Compression Machinery for Oil and Gas, 2019

Process Control With Centrifugal Compressors Driven by Two-Shaft Gas Turbines

The following is a description of a typical control scenario, in this particular case for compressors with a gas turbine driver that can operate at variable speeds.

Centrifugal compressors, when driven by two-shaft gas turbines, are usually adapted to varying process conditions by means of speed control. This is a very elegant way of controlling a system, because both the centrifugal compressor and the power turbine of a two-shaft gas turbine can operate over a wide range of speeds without any adverse effects. A typical configuration can operate down to 50% of its maximum continuous speed, and in many cases even lower. Reaction times are very fast, thus allowing a continuous load following using modern, Programmable Logic Control (PLC)-based controllers.

A simple case is flow control: the flow into the machine is sensed by a flow metering element (such as a flow orifice, a Venturi nozzle, or an ultrasonic device). A flow set point is selected by the operator. If the discharge pressure increases due to process changes, the controller will increase the fuel flow into the gas turbine. As a result, the power turbine will produce more power and cause the power turbine, together with the driven compressor, to accelerate. Thus, the compressor flow is kept constant. Both the power turbine speed and the power increase in that situation.

If the discharge pressure is reduced, or the suction pressure is increased due to process changes, the controller will reduce the fuel flow into the gas turbine. As a result, the power turbine will produce less power and cause the power turbine, together with the driven compressor, to decelerate. Consequently, the compressor flow is kept constant.

Similar control mechanisms are available to keep the discharge pressure constant, or to keep the suction pressure constant. Another possible control mode is to run the unit at maximum available driver power (or any other constant driver output). In this case, the operating points are all on a line of constant power, but the speed will vary.

The control scheme works for one or more compressors, and can be set up for machines operating in series, as well as, in parallel.

If speed control is not available, the compressor can be equipped with a suction throttle, or with variable guide vanes. The latter, if available in front of each impeller is rather effective, but the mechanical complexity proves usually to be prohibitive in higher-pressure applications. The former is a mechanically simple means of control, but it has a detrimental effect on the overall efficiency.

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Industrial control engineering

Peng Zhang , in Advanced Industrial Control Technology, 2010

(2) Motor power transformation

In motion control systems, motors are the components that convert electrical power to useful mechanical power. Gearheads transform the mechanical rotary power to the desired combination of speed and torque if the motor cannot do so directly. Transmission systems are chosen on the basis of performance, size, cost, and various other constraints. Whilst each type of system has its own benefits and disadvantages, gearing systems hold many advantages for economic power consumption in small packages.

There are several types of electrical motors and power transmission devices. Depending on the application, a designer might first choose between alternating current (AC) and direct current (DC) motors, then between styles (including induction, universal, and permanent magnet), and finally between the many types specific to each style. The designer must also select a controlling device and amplifier to provide the proper input power for the motor. For example, the motor may be capable of supplying the necessary power, but not match the speed or torque requirements. Most motors are rated to operate for peak output around a certain continuous load range of torque and speed. To operate the motor outside that range would either under-utilize the motor and result in an oversized motor, or over-utilize the motor and potentially run the risk of damage motor. Although each type and style of motor operates differently, smaller motors typically operate more efficiently at higher speeds and lower torques. Larger motors operate at higher torques and lower speeds. This is obvious if the motor is viewed as a large moment arm.

High-energy, permanent magnet, servo motors have become extremely popular industrial motion controls. For these motors, a general rule of thumb is that if the length of the motor is doubled, then the output torque capacity of the motor is also doubled. However, the output torque capacity of the motor will be quadrupled if the length is kept the same and the diameter of the motor is doubled. For maximum efficiency, proper sizing, and economy, it is best to design the motor for operation at, or slightly under, its designed load point of speed and torque.

The speed-reducing planetary gearhead as described adheres to the following:

ratio = (Ns + Nr)/Ns (where Ns = number of sun gear teeth and Nr = number of ring gear teeth).

There are many variations of epicycle gearing that can be produced from this style. In some instances, the ring gear can be the driving gear or the output gear. Complex epicycle, or planetary gearing is also possible, in which gear clusters are used in planetary configurations. An automobile transmission is a good example of how several styles of planetary gearing can be utilized together.

To enhance the output of such a spur gear design, a style of involuted spur gearing, known as epicycle gearing (i.e., planetary gearing), was developed. Planetary gearing dramatically increased the strength of the gearhead while eliminating radial loading, by counterbalancing the effect of any one-gear engagement. Planetary gearheads distribute the input power coming from the sun gear (input pinion) to two or more planetary gears. Since the planet gears are positioned symmetrical to the pinion gear, radial loading on the pinion gear is eliminated. The planet gears are housed within a ring gear, which has gear teeth cut into the inside diameter in a reversed tooth profile. The separating forces (radial forces) produced from the planet gears' tooth engagement with the sun and ring gears cancel each other. The ring gear is stationary, and the input sun gear pinion drives the planet gears, which then walk in the same rotational direction as the sun gear.

Depending on the number of teeth in the sun gear and ring gear, each stage typically generates speed reduction ratios of between 3 and 10 times the input speed. A two-stage planetary gearhead, which has two 10-to-1 stages (i.e., 10 rotations of the input pinion results in one rotation of the output), yields 100 times the output torque (less efficiency) and 1/100 the rpm. If the gearhead were reversed so that the output shaft becomes the input shaft, the speed would increase by 100 times; the available output torque would decrease proportionately.

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Batteries in PV Systems

David Spiers , in Practical Handbook of Photovoltaics (Second Edition), 2012

10.5 Examples of Predicted Battery Lifetimes

The depth of daily cycling varies considerably between different types of PV systems. In systems with a large autonomy reserve, such as in telecommunications, the cycling will be quite shallow, whereas in systems with less autonomy, such as lighting systems, the daily cycling will be higher. As shown, earlier, the typical range of daily cycling is of the order of 2–30%, except in some hybrid PV-diesel systems, where it can be much deeper. Similarly, there can be quite a wide range of operating temperatures experienced by batteries in PV systems, depending on the climate at the site and also the nature of the battery enclosure. For predicting the lifetime limit due to corrosion, the annual average battery temperature should be considered, and in a well-designed system this should be close to (probably a few degrees above) the average annual ambient temperature.

Figure 11 illustrates the different limits for typical PV systems of different types. We plot the corrosion-limited lifetime against battery temperature, and also display the cycle limit for different numbers of days of autonomy on the same graph.

Figure 11. Predicted corrosion and cycle life limits in different types of PV system. (a) Professional PV system with continuous load, tubular batteries. (b) PV rural lighting system with flat-plate batteries. (c) Vaccine Refrigerator CFS, tubular batteries

The upper graph shows the situation in a 'professional' PV system with a continuous load and tubular-plate batteries, typical of a remote telecommunications or cathodic protection PV system. The number of days of autonomy required for such systems is normally at least 5, and is often much higher. The combination of the shallow cycling and the good cycle life of the tubular-plate battery means that the cycle life hardly ever limits the battery life at operating temperatures above 20°C. Instead, the corrosion limit of 12 years at 20°C, 6 years at 30°C, or 3 years at 40°C is what normally applies for the battery life in such systems.

The middle graph illustrates a much different case, typical of many rural lighting PV systems. For reasons of cost, a flat-plate battery is often specified. A good but low-cost flat-plate battery may give around 200 deep (80%) cycles, and this is what is illustrated in this graph. Daily cycling is considerably deeper in such a system than in the first example, not just because less days of autonomy are usually specified for such systems (2–5 is typical), but also because all the discharge occurs at night. Now we see that the predicted battery life is around 1 year for 2 days autonomy, whatever the temperature, around 2 years for 3–4 days autonomy at temperatures below about 32°C and around 3 years for 5 days autonomy at temperatures below about 30°C. Above these temperature limits, the corrosion limit applies, giving an even shorter life. If a normal car battery is used in such a PV system, it will give even less life than shown in the graph, due a shorter cycle life than the example battery. The message here is quite clear—relatively cheap batteries with a short cycle life can give a very disappointing life in PV lighting systems, especially if a low capacity (3 days autonomy or less) is also specified.

The lower graph shows the interesting case of a PV vaccine refrigerator system with tubular-plate batteries. Here the daily load, and thus the daily depth of discharge, will vary with the ambient temperature (the refrigerator consumes more electrical energy if it is working in a hot location), and will also vary according to how it is used (e.g., if ice packs are frozen regularly, it will consume more than if it is just storing vaccine. Early examples of PV vaccine refrigerator systems were often installed in some very hot locations, and the resulting battery life was disappointing (less than 5 years), even though good-quality tubular-plate batteries were used. The initial reaction was to look for batteries with a better cycle life—e.g., traction batteries—but the graph clearly shows that it is not the cycle life but the temperature-dependent corrosion that is causing the short battery life at high operating temperatures. Siting the battery in a cooler location is the answer to obtaining a longer battery life in this case.

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Anthropogenic Trace Compounds (ATCs) in aquatic habitats — Research needs on sources, fate, detection and toxicity to ensure timely elimination strategies and risk management

Sabine U. Gerbersdorf , ... Silke Wieprecht , in Environment International, 2015

9 Conclusions

The continuous load to aquatic systems by ATCs has led to numerous activities in research, application and legislation, whether based on ecotoxicological evidence or acting on the precautionary principle. Up to now, most resulting publications have a certain perspective (e.g., from the chemical analytic point of view) and focus on one group of substances (e.g., endocrine disrupting compounds) or one specific environment (e.g., wastewater treatment plants). This review illustrates the need for a multi-disciplinary effort that addresses crucial questions on ATC occurrence, fate, detection, toxicity, elimination and risk assessment from source to sink. Thereby we consider the human impact on ATC entry and distribution as well as the potential impairment of the environment and human health by ATCs. With the broader view on both natural and technical aquatic systems, covering different scales and involving fundamental as well as applied science, we aim to contribute to a timely, innovative and holistic research design for ATCs in aquatic systems.

In order to get a comprehensive picture on ATCs' occurrence and fate, our review acknowledges briefly the essential improvements in the sensitivity of chemical analysis. Equally important are sophisticated monitoring campaigns that are based on internationally validated sampling guidelines (e.g., representative locations, frequency and type of sampling) and methods for analysis and data evaluation. In proper monitoring and analysis, the boundary conditions, interactions between compartments as well as periodic or episodic variations have to be considered for a better large-scale comparability of ATC data. To meet the challenges by the daily growing numbers and complexity of ATCs, we postulate a future focus on indicator substances that represent chemical classes with similar physico-chemical properties and, thus, similar characteristics of solubility and persistence. Appropriately chosen indicators can describe specific introductory pathways as well as transport behaviour and final sinks for certain ATC classes. In this context, a paradigm shift is required in such that the indicators should not be chosen by their toxicity.

Nevertheless, knowledge on toxicity is vitally important since this is the basis to reduce or substitute ATCs by legal enforcement, identify locations in urgent need of action and verify the successful implementation of prevention or elimination strategies. Despite much progress in both bioanalytics and biomonitoring, new test systems have to evolve and to be harmonized to better assess on various toxicity levels (from gene to whole organism, from bacteria to vertebrates, from community to environment). The big challenge ahead is to comprehensively investigate a highly complex system of intra- and interrelations using laboratory – and thus simplified – approaches, and still understand what the findings mean on an environmental level. We also highlight the urgent need to extrapolate from well-known acute toxicity to long-term effects on environmental and human health.

Our review further concerns the exposure paths of ATCs and identifies the WWTPs as a main pathway, whether directly or indirectly, while emphasizing the problems associated with surface runoff, sewer basin overflow as well as the recycling of biowaste and biosolids. Briefly, the status quo and challenges for current physical, chemical and biological ATC elimination techniques are presented. While new technologies such as ozonation or activated carbon seem to be quite effective in ATC removal, the interactions between ATC type, boundary conditions and dosage are not entirely understood although they largely determine removal success. Bioaugmentation seems to be a promising alternative for investing additional research; however, finding the right microbial consortia to degrade substances in low and fluctuating concentrations still poses a challenge, from laboratory level up to technical implementation. The benefit of these locally acting end-of-pipe strategies is then opposed to what should be the top priority for larger-scale solutions: avoidance strategies.

ATCs are released into the environment where they can accumulate, as previous research has shown. Despite this fact, there is surprisingly little information on the fate of ATCs in natural habitats. Particle-associated ATCs might couple their fate closely to the dynamic of fine sediments that, in turn, is very much influenced by (micro-) biological activity. New findings on the complex interrelation between microbial secreted EPS and cohesive sediment stability are presented that also point to the crucial role of biofilm for sorption and degradation of non-polar ATCs. This might even apply for polar ATCs that, after travelling unhindered through the water body, can eventually enter the subsurface (e.g., soils, the hyporheic zone and groundwater) where small-scale pore geometry encased by organic material might substantially complicate transport and attenuation processes. Altogether, this illustrates the essential role of biofilms in ATCs fate by changing sediment stability and sediment entrainment (a phenomenon called biostabilization), subsurface porosity and permeability as well as sorption and degradation capacity of sedimentary compartments.

Last but not least, the necessary steps and the importance of a comprehensive risk assessment for ATCs are demonstrated in order to assist the "source to tap" approach in implementation and evaluation of regulative policies and management directives. Finding new ways towards a holistic research design for ATCs is essential, especially when regarding the future challenges in water allocation and water quality in terms of demographic (9 billion humans in 2050, longer life-spans) and global changes (weather extremes, unprecedented variance in the precipitation regime) as well as ongoing globalization (intensified and unsustainable use of water resources) (IPCC, 2012).

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Waste Biorefinery - Advocating Circular Economy

Jonathan Winfield , ... Ioannis Ieropoulos , in Bioresource Technology, 2016

2.2.4 Stacking ceramic MFCs

As discussed in Section 2.2.1 ceramic material can provide a stable environment for the bacterial contingent; additionally its properties can also be utilised, making it suitable for more efficient energy harvesting. This was demonstrated using stacks of ceramic MFCs connected in parallel compared to conventional IEM MFCs (Walter et al., 2014 ). The ceramic stack displayed a capacitive-like behaviour when subjected to intermittent loading, i.e. switching between open circuit and closed circuit conditions. This resulted in significantly higher power bursts than when operated under continuous load, and it confirmed that ceramic membrane MFCs can provide a more efficient platform for operating energy harvesting and power management systems.

For scaling up MFCs, stacks need to be employed consisting of a number of units connected together electrically. Many studies were conducted using MFCs with conventional membranes, which demonstrated that series–parallel might be the optimal electrical configuration (Ieropoulos et al., 2008). Recently there have been a number of studies employing ceramic MFC stacks. For example a multi-electrode MFC system represented a scaled-up ceramic system using 3 anodes in a large 26L chamber (Ghadge and Ghangrekar, 2015b), which demonstrated that litre-scale ceramic MFCs can operate on a long term basis (14   months). Previous work has demonstrated that a plurality of smaller units improves power density (Ieropoulos et al., 2008) and so the large chamber might be optimised using a more efficient configuration.

Using the smaller sized stacks of cylindrical MFCs with internal anodes, an exciting demonstration of practical implementation was demonstrated where, via energy harvesting, a mobile phone was charged using urine as the feedstock (Ieropoulos et al., 2013). Following on from this, a multi-electrode system incorporating small scale MFCs with internal cathode configuration demonstrated improved efficiency in charging mobile phones (Gates Foundation, 2014). Ongoing work has continued to improve efficiency through the development of innovative designs. For example, novel ceramic-based stacks have been developed using membrane-less systems. Utilising multiple modules, each consisting of stacks of closely packed terracotta plates, the efficient utilisation of urine was demonstrated in the setup whereby the insulation between cathode and anode was driven by the self-stratification of the urine (Walter et al., 2015b).

The combination of ceramic MFCs and urine as fuel has helped push MFC technology closer to deployment in real life applications. For example, prototype urinals have now been trialled on a University campus and at Glastonbury music festival (Ieropoulos et al., 2016). In both cases (field trials), energy generated by the "Pee Power" system was sufficient to operate motion sensors, light LED modules and perform efficient urine treatment. Over 400 inexpensive ceramic MFCs were used in the festival urinal and the success of these trials demonstrates that the deployment of ceramics in MFCs can help push the technology towards real world applications. The use of ceramics in microbial fuel cells is still relatively new and despite the progress made, there still are some challenges to be overcome, as will be discussed next.

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A review on the factors influencing biohydrogen production from lactate: The key to unlocking enhanced dark fermentative processes

Octavio García-Depraect , ... Germán Buitrón , in Bioresource Technology, 2021

4 The occurrence of lactate-based hydrogen production

Even though organic waste and wastewater are complex substrates, their physicochemical and biological characteristics appear to be somewhat related to the occurrence of lactate-based hydrogen production. Table 1 displays a summary of different complex substrates that have been reported to sustain hydrogen production from lactate; the most widely reported being cheese whey, food waste and vinasses, respectively. On a closer inspection, although it is evident that each of those substrates has its own unique physicochemical and microbial population composition, they share many common features, as discussed in this section.

Table 1. Features of feedstocks and bioprocesses sustaining the lactate-driven hydrogen production.

Substrate pH/tCarb/lactate Operational conditions Inoculum (source)/pretreatment or enrichment Performance Reference
Beer lees 4.1/NR/1.6 g/L 125-mL batch flask reactor, 37 °C, initial pH 7.0 (uncontrolled conditions) Beer lees, sewage sludge or garbage compost/No pretreatment 13.9 to 23.9 NL H2/kg TS Bando et al. (2013)
Tequila vinasse 3.6/19.6 g/L/NR 3-L well-mixed batch reactor, 35 °C, 500 rpm, two-stage pH-shift control from 6.5 to 5.8, nutrient supplementation Mixed culture PTA-124566 124 NL H2/kg VSadded, 3.8 NL H2/L-d, 65–70% (v/v) H2 content García-Depraect and León-Becerril (2018)
Tequila vinasse 3.6/18.7 g/L/2.5 g/L 3-L well-mixed batch reactor, 500 rpm, two-stage pH-shift control from 6.5 to 5.8, nutrient supplementation Mixed culture PTA-124566 115.6 NL H2/kg VSadded, 2.7 NL/L-d Diaz-Cruces et al. (2020)
Sugarcane vinasse 4.5/a4.5 g/L/NR Continuous bench-scale AnPBR, 55 °C, pH 5.1–5.2, HRT 7.5 h, OLR 84.2 g COD/L-d Sugarcane vinasse/ Natural fermentation under 55 °C 0.6–1.1 NL H2/L-d Fuess et al. (2018)
Sugarcane vinasse 4.6/5.0 g/L/1.7 g/L 1.9-L AnSTBR, 55 °C, pH 5.2, HRT 12 h, OLR 90.0 g COD/L-d Sugarcane vinasse/ Natural fermentation under 55 °C 2.1 NL H2/L-d, 1.0 ± 0.2 mol H2/kg CODfed Fuess et al. (2019)
Sugarcane vinasse 4.6/NR/2.1 g/L 0.5-L batch flask reactor, 55 °C, initial pH 6.5 (uncontrolled conditions) Sugarcane vinasse/ Natural fermentation under 55 °C 2.23 mol H2/kg COD Couto et al. (2020)
Unfermented garbage slurry 5.01/NR/1.4 g/L 1.3-L AnSBR, 37 °C, 90 rpm, pH 6.0, HRT 48–14 h Leaf-litter cattle-waste compost/No pretreatment 20.5 NL H2/kg VSadded, 20.8% (v/v) H2 content Ohnishi et al. (2010)
Food waste NR/71.53 g/Kg food waste/NR 1.5-L AnSBR, 35 °C, pH 5.5, HRT 48 h Anaerobic granular sludge/Thermic 6.0 mmol H2/L-d, 23.2% (v/v) H2 content Santiago et al. (2019)
Cassava starch wastewater 4.9/6.4 g/L/1.6 g/L 6.0-L AnSBBR, fed-batch, 30 °C, 300 rpm, initial pH 6.0 (uncontrolled conditions) Anaerobic sludge/Thermic (95 °C, 15 min) 2.4 L H2/L-d, 3.7 mol H2/kg tCarb, 31% (v/v) H2 content Andreani et al. (2019)
Cheese whey 6.2/b57.7 g/L/NR 2.0-L well-mixed batch reactor, 39 °C, 150 rpm, pH 6.5 Cheese whey/No pretreatment 111.6 NL H2/kg TOC, 10.3 NL H2/kg TOC-h, 65% (v/v) H2 content Asunis et al. (2019)
Synthetic cheese whey NR/76.5%w/w/– 1.6-L AnSTBR, 25 °C, pH 5.1, HRT 24 h, OLR 24 g COD/L-d Natural fermentation of glucose-based synthetic wastewater 1.6 ± 0.7 L H2/L-d, 1.4 ± 0.7 mol H2/mol lactose consumed Blanco et al. (2019)
Milk (93%), yogurt (5%) and fresh mild whey cheese (2% w/w) 6.2/*47.2 g/L/NR 0.75-L CSTR, 37 °C, pH 5.0, HRT 6 d Hydrogen-producing reactor treating olive mill wastewater (55%), cheese whey (40%) and liquid cow manure (5% v/v) ~0.8 NL H2/L-d, 0.84 mol H2/mol tCarb or 40.6 L H2/kg VSadded or 24.3 L H2/kg CODadded, 28.7% (v/v) H2 content Stavropoulos et al. (2016)
Tapioca starch bioethanol fermentation residues 4.0./22.6 g/L/5.0 g/L 12-L CSTR, 35 °C, 160 rpm, pH 6.0, HRT 8.2 h, OLR 117.6 g COD/L-d Hydrogen-producing reactor treating food wastes/Not reported 2.6 NL H2/L-d, 22.0 L H2/kg COD, 38% (v/v) H2 content Juang et al. (2011)
Rice straw bioethanol fermentation residues NR/5.1 g/L/3.7 g/L 5-L CSTR, 35 °C, pH 6.0, HRT 12 h, OLR 46 g COD/L-d, nutrient supplementation Hydrogen-producing reactor treating food wastes/Not reported 0.9 NL H2/L-d, 20.0 L H2/kg COD, 38% (v/v) H2 content Cheng et al., (2012)
Oil-extracted microalgae residues NR/55.7 g/L/NR 1-L batch flask reactor, 37 °C, pH 5.5, 15 g tCarb /L and nutrient supplementation Hydrogen-producing reactor treating food wastes/Not reported 66.1 NL/kg tCarb Cheng et al., (2016b)
Bagasse bioethanol fermentation residues NR/8.7 g/L/3.7 g/L 5-L CSTR, 37 °C, 160 rpm, pH 6.0, HRT 4 h, OLR 180 g COD/L-d, nutrient supplementation Hydrogen-producing reactor treating rice straw bioethanol fermentation residues /Not reported 0.9 NL H2/L-d, 4.8 L H2/kg COD, 38% (v/v) H2 content Cheng et al. (2016a)
Sugar beet molasses NR/ NR/ NR 3-L AnPBR, room temp., HRT 18 h, 54 g molasses/L, pH ~ 5.0, nutrient supplementation Eutrophic, meromictic lake/Thermic 4.4 L H2/L-d, 0.8 mol H2/mol tCarb consumed, 48.6% (v/v) H2 content Chojnacka et al. (2011)
Sugarcane molasses NR/564.7 g/L/17.9 g/L 1.5-L EGSB, 30 °C, 15 g sucrose/L, pH 4.0–4.7, HRT 1 h, OLR 360 g COD/L-d, superficial velocity 6 m/h, nutrient supplementation UASB reactor treating poultry slaughterhouse wastewater treatment/ Thermic 13.9 L H2/L-d, 0.13 mol H2/mol hexose, 13.0 ± 2.4% (v/v) H2 content Freitas et al. (2020)
Molasses 4.3/614 g/kg/NR 2.5-L AnPBR, 35 °C, HRT 6 h, pH 5.5, molasses (28 g COD/L) Anaerobic digester sludge/Thermic 2.8 L H2/L-d Park et al. (2010)
Food waste 6.8/13 g/L/3.9 g/L 1-L batch reactor, 55 °C, 600 rpm, pH 6.0 Kitchen waste compost/ No pretreatment 0.48 mol H2/kg COD, 40% (v/v) H2 content Lee et al. (2008)
Vegetable kitchen waste NR/25 g/L/5.5 g/L 20-L AnSBR, 55 °C, 160 rpm, HRT 96 h, OLR 28 g COD/L-d, initial pH 6.0 (uncontrolled conditions) Kitchen waste compost/ No pretreatment 1.0 L H2/L-d, 1.7 mol H2/kg COD, 40% (v/v) H2 content Lee et al. (2010)
Co-fermentation of olive mill wastewater (55%), cheese whey (40%) and liquid cow manure (5% v/v) 5.0 (OMW), 5.7 (CW) 7.3 (LCM)/30.3 g/L (OMW), 53.6 g/L (CW), 8.1 g/L (LCM)/NR 1-L well-mixed batch reactor, 37 °C, 150 rpm, pH 6.0 Anaerobic acidogenic CSTR treating the same mixture of OMW, CW and LCM/No pretreatment 0.6 mol H2/mol equivalent glucose consumed Dareioti et al. (2014)
Co-fermentation of fruit and vegetable waste (50%) and raw cheese whey (50% v/v) 5.3 (FVW), 4.5 (CW), 0.6 g/L (FVW), 28.9 g/L (CW)/NR 2-L batch flask reactor, 37 °C, 150 rpm, pH 5.5 Anaerobic digester treating fruit and vegetable wastes/not reported 256.3 mmol H2/L-d, 449.8 NL/kg COD Gomez-Romero et al. (2014)

The occurrence of lactate-based hydrogen production has been triggered using substrates hosting LAB as part of their indigenous microbial population, which typically contain relatively high amounts of lactate and acetate. For instance, Andreani et al. (2019), reported Lactobacillus, Lactococcus and Leuconostoc in cassava starch wastewater, which was characterized by lactate and acetate concentrations up to 4.1 g/L and 0.7 g/L, respectively. In sugarcane molasses, lactate and acetate concentrations averaged 17.9 ± 8.9 g/L and 23.8 ± 3.2 g/L, respectively (Freitas et al., 2020 ). LAB can be either originally present in the substrate or proliferate during storage and transportation. Therefore, substrates themselves can be a direct source of LAB, which would entail a high and continuous load of LAB in the system. Eventually, this may provoke the over-proliferation of LAB and consequently, the process might exhibit impaired hydrogen production rate and yield. Most probably, this fact makes lactate-based hydrogen production the only potential strategy to overcome the technical problems of DF processes related to LAB overgrowth.

From a retrospective perspective, the physicochemical characteristics of substrates also constitute a selection pressure that shapes the microbial community structure and its associated metabolic patterns in DF processes. For example, hexoses are more selective for LAB, whereas pentoses are more conducive to HPB growth (Chatellard et al., 2016). Indeed, LAB are well-known colonizers of aerial parts of many plants, the gastro-intestinal tract of animals and humans as well as different locations of the human body (Douillard and de Vos 2014). These autochthonous microbiotas have constituted a reservoir for LAB in traditional fermentation based on raw materials derived from milk, meat and plants. In other words, carbohydrate- and protein-rich substrates provide a good environment for LAB growth. Even if most of the LAB are auxotrophic for many amino acids and vitamins (e.g., biotin, histidine, methionine) (Teusink and Molenaar, 2017), they can take up growth factors from the substrate and/or from the surrounding microorganisms and fermentation medium. Hence, substrates derived from fermented dairy products (e.g., cheese, yogurt), meat, fruit, vegetable and plants rank among the most suitable substrates to produce hydrogen via the lactate-type fermentation. In this regard, substrates sustaining the production of hydrogen from lactate include vinasses, beer lees, food waste, organic fraction of municipal solid waste (OFMSW), dairy by-products, cassava wastewater, bioethanol fermentation residues, among others (Table 1).

LAB comprise a heterogenous group of Gram-positive bacteria, Lactobacillus, Lactococcus, Streptococcus, Leuconostoc, Pediococcus, and Enterobacter that belong to the phylum Firmicutes, but also, the phylum Actinobacteria represented by the genus Bifidobacterium (de Vos 2011). Many LAB (e.g., Lactobacillus, Sporolactobacillus) can metabolize hexoses, pentoses and disaccharides (e.g., glucose, galactose, arabinose, ribose, xylose, cellobiose, lactose, maltose, sucrose) to lactate or to lactate, carbon dioxide and acetate or ethanol, or to lactate and acetate, depending on the prevailing Embden-Meyerhof-Parnas (homolactic), phosphoketolase (heterolactic) or bifidum pathway, respectively (Asunis et al., 2019; Dareioti et al., 2014). Similarly, some LF-HPB such as C. beijerinckii are able to ferment not only lactate but also a wide variety of compounds such as glucose, fructose, galactose, mannose, xylose, arabinose, lactose, sucrose, maltose, glycerol, among others (Bhat and Barker, 1947). However, auxotrophic LAB often outcompete prototrophic fermenters such as Clostridium by having a higher growth rate and/or more rapid substrate uptake in complex environments (Rombouts et al., 2020). This competitive advantage of LAB over other bacteria including HPB could be attributed to the loss of biosynthetic functions in LAB (Rombouts et al., 2020; Teusink and Molenaar, 2017). The pan-genome of LAB is small compared to other bacteria consisting approximately 14,000 genes and a core genome of 383 orthologous genes (Kant et al., 2011). The common ancestor of LAB lost several genes required for the synthesis of various co-factors resulting in the present members that require complex growth media present in raw substrates (Baureder and Hederstedt, 2013). However, horizontal gene transfer drives the genomic diversity and plasticity in LAB by the gain of new genes due to ecological adaptation. These mobile elements encode important industrial characteristics such as transporters to optimize the nutrient uptake, proteinase, bacteriocin and exopolysaccharide production, immunity against bacteriophages, resistance to acid-shock stress, detoxification-related functions of solubilized components of lignin and furfural/hydroxymethylfurfural, ability to switch from fermentation to aerobic respiration metabolism when they are grown on heme-containing media, and pili production involved in biofilm production (Lee and Pi, 2010; de Vos, 2011; Douillard and de Vos, 2014). All these genomic features unique to LAB reveal how these bacteria adapt and dominate food fermentations through a three-phase ecological evolution strategy (Ercolini et al., 2013). The ecological advantage of LAB may provide a plausible explanation of why hydrogen production batchwise from lactate can occur in two consecutive stages, depending on the prevailing microbiome and the type and properties of the substrate(s).

Using a mixture of tequila vinasse and nixtamalization wastewater, García-Depraect et al. (2019b) observed that fermentable sugars were converted into lactate with little or no concomitant generation of acetate and carbon dioxide by the action of LAB in a primary lactate fermentation, while LF-HPB metabolized the produced lactate, typically together with acetate, to produce mainly hydrogen, butyrate and carbon dioxide in a secondary lactate fermentation. This two-stage lactate fermentation has also been observed for raw (Asunis et al., 2019) and synthetic (Blanco et al., 2019) cheese whey, bioethanol residues from rice straw (Cheng et al., 2012), bagasse (Cheng et al., 2016a) and tapioca starch (Juang et al., 2011); tequila vinasse (García-Depraect and León-Becerril, 2018), food waste (Dauptain et al., 2020), olive mill wastewater (Mugnai et al., 2021), and in the co-fermentation of olive mill wastewater, cheese whey and liquid cow manure (Dareioti et al., 2014) and of cheese whey and fruit and vegetable waste (Gomez-Romero et al., 2014). This aspect not only shows that hydrogen production can be uncoupled from carbohydrate consumption but also provide evidence that enough carbohydrates in the substrate (sugar-rich substrates) are needed to supply lactate and acetate as the hydrogen precursor, albeit the lactate (and acetate) initially contained in the substrate can also be used to produce hydrogen. However, it should be noted that hydrogen can be in practice produced from the combined use of carbohydrates and lactate and acetate. Previous studies have confirmed that both the physicochemical and the microbial substrate features are factors that can determine how frequently lactate-based hydrogen production occurs in DF reactors. At this point, it should be stressed that besides the compositional features of the substrate, the lactate-based hydrogen production efficiency is also heavily dependent on other interrelated factors such as the operational conditions, bioreactor design, inoculum source and pretreatment/enrichment method, as discussed in detail in the following sections.

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