A powerful suction vacuum is used to only remove the dry, very light shells from the hatchery waste, leaving the heavier infertile eggs (World Intellectual Property Organization-WO/2001/074491, Eggshell waste processing method and device). The shell and non-shell materials can also be separated by using a vibrating or shaking device (e.g., a shaker-sieve belt), which can separate lighter parts from heavier parts in the hatchery waste. A stream of gas (such as a cyclone forced-air separator) also can be used to separate lighter materials from heavier materials in hatchery waste. After hatching, live chicks and unhatched chicks or clear eggs from the hatching tray are placed on a moving belt with fixed gaps that only allow chicks to slide through, while shells and unhatched eggs are retained on the belt. Then the shell is vacuumed up for further separation, with the dead embryos being disposed into a separate container (World Intellectual Property Organization-WO/2001/074491, Eggshell waste processing method and device).

Eggs shells can be composted with other organic materials to increase the mineral content of the compost. Other minor uses for crushed egg shells include; 1) spread around plants to deter slugs and snails, 2) mixed with garden soil for use as a fertiliser; 3) fine pieces of crushed egg shell mixed with seeds for use as a feed for aviary birds; 4) added to cement to increase its strength; 5) used by artists to make mosaics and; 6) to make textured paint for 3D effects in artwork. These are small niche uses and not suited to industrial volumes of egg shells [20,21].

Complete separation of the membrane and the shell increases the value of the resulting products. One method is to use a meat processing machine to grind egg shells into a powder, and then mix the powder with water to separate the membrane. The shell sinks and the membrane stays suspended in the water. Another method is to place the egg shells into a tank containing a fluid mixture and use cavitation (vapour bubbles in a flowing liquid) in the fluid mixture to separate the shell membrane. The fluid mixture is ideally recirculated to continue the separation process [22]. Egg shell membrane contains about 10% collagen, which can be used in medicine. The market price for purified collagen is US$1000 per gram. Collagen glue is used as a filling in corneal wounds. It can be also used for skin grafts, dental implants, angioplasty sleeves, plastic surgery and treatment of osteoporosis and in pharmaceuticals as well as food casings and film emulsions. The pure shell powder can be used in the paper industry, or as a lime substitute [23] or calcium supplement in agriculture [24].

Other uses of egg shell membrane include:

Use as a component of biodegradable plastic.

The hatchery waste can be automatically fed by conveyor belts into a furnace which is equipped with a rotating shredder unit for chopping and grinding solid waste. An incinerator system can be used as a furnace to heat the solid and liquid waste to produce steam. The steam can power a turbine generator to produce electricity [25]. The use of a steam turbine at a hatchery may only be economic if the hatchery is producing a large volume of waste.

The rendering process simultaneously dries the material and separates the fat from the protein [26] and yields fat and a protein meal (e.g., hatchery waste by-product meal) similar to meat and bone meal or fertiliser [27]. There is shortage of protein meals world wide for use in diets particularly in the pig and poultry industries. The decision to render hatchery waste depends on whether it is cheaper to transport the waste to a rendering facility or to send to landfill. A major issue when using by-products such as hatchery waste meal in diets is whether they are pathogen free.

Extruded or autoclaved hatchery waste could be used as livestock feed. Said [28] reported that dry extrusion was an effective method to treat hatchery waste. For example Miller [29] extruded a mixture of ground hatchery waste and yellow maize meal (25:75) at 140 °C for 10 s, while Lilburn et al. [30] autoclaved turkey hatchery waste for 15 min at 125 °C and 1.76 kg/cm² and then dried it at 50 °C for 1 hour. Similarly Verma and Rao [31] autoclaved hatchery waste (infertile eggs or eggs with dead embryos) and dried it at 100 °C for 10 h. Ravindra-Reddy and Rajasekhar-Reddy [32] autoclaved day old cull male chicks for 30 min. The product was dried and powdered and used as poultry feed.

Hatchery waste could be treated in the same way as poultry waste (feathers, heads, feet and inedible entrails (intestine, lung, spleen) by boiling at 100 °C with a pressure of 2.2 kg/cm² for 15 min; then boiled again at 100 °C for 5 hours, followed by boiling at 130 °C for 1 h then cooled to ambient temperature [33]. Likewise dead embryos could be boiled for 100 °C for 30 min, soaked in cold water for 20 min to remove shells, sun dried for 4d and used in poultry feed [34]. Cooking hatchery waste with water (2:1) then dehydrating to a dried product has been used as livestock feed [35-37]. Nutritive value of the dried dead embryos is 36% CP, 27% ether extract, 17% ash, 10% calcium and 0.6% phosphorus [38].

Kompiang [39] reported a method of ensiling rejected hatchery eggs. The eggs were mixed in a 1:1 ratio with formic and propionic acids for 8 weeks at room temperature. Formic acid is suitable for the ensiling of materials such as wet and protein-rich resources. Propionic acid and formic acid have been used to preserve and ensile non fertile eggs and dead embryos. The acids act by intervening specifically in the metabolism of the microorganisms involved in spoilage. In addition, the reduction in the pH creates an environment which is unfavourable for microorganisms. The rapid reduction in the pH diminishes the growth of bacteria which produce butyric acid and ammonia and promotes the growth of lactic acid-producing bacteria. The lactic acid is responsible for the low pH necessary for storage of the by-product before being used in animal feed.

Kim and Patterson [40] treated culled birds for 12 h at 21 °C with 25.6 mg of INSTAPRO enzyme or 2 h at 21 °C in 0.4 N NaOH. The resulting product was fermented (with added sugar) for 21 days. After fermentation the products were autoclaved at 124 kPa and 127 °C for 90 min, then dried in a forced-air oven at 60 °C and the final product was used as poultry feed [40]. The advantage of the enzyme or NaOH treatment is that nutrients in poultry meal are more readily digestible by birds and improve the availability of essential amino acids in the treated meal.

Composting is a common method for solid organic waste disposal [41,42]. In this process, mesophilic and thermophilic micro-organisms convert biodegradable organic waste into a value added product [41,43,44]. The decomposition of organic waste is performed by aerobic bacteria, yeasts and fungi. The composting process kills pathogens, converts ammonia nitrogen to organic nitrogen and reduces the waste volume [41]. The product can be used as a fertiliser. Disadvantages of composting are loss of some nutrients including nitrogen, the land area required for the composting and odour problems. Das et al. [1] reported that composting hatchery waste with sawdust and yard trimming in a ratio of 3:2:1 or composting it with sawdust, yard trimmings and poultry litter in a ratio of 2:1:1:2 eliminated 99.99% of E. coli. Composting with litter also eliminated Salmonella, but Salmonella was present if temperature was too low.

When hatchery waste is composted with poultry litter it will produce a safe and rich organic product which is a good organic fertiliser. It is important to control the moisture content and keep raising the temperature of the compost to eliminate the pathogens. Composting hatchery waste with poultry litter produces a product that contains 1% nitrogen, 2.5% phosphorus and 0.25% potassium on a dry weight basis. The product also contains high calcium and other micro-nutrients [45].

A potential method for treating hatchery waste on a hatchery site is to use an ‘in-vessel’ composting technique to decompose and stabilize the un-separated hatchery waste obtained directly from the hatchery. The hatchery waste can be mixed with wood shavings to reduce the moisture then composted [14]. There are a number of ‘in vessel’ composters on the market that could be used for stabilising hatchery waste. The composter turns manure, litter, sour feed stuffs and carcasses into compost in 4 days with minimal labour and mechanical devices [46].

While anaerobic digestion has not been used to treat hatchery waste it is the most popular process used to treat human effluent and other livestock waste. It has the advantage of being a high efficiency process and produces biogas [48] for power generation or heating. The Poultry Industry has overlooked using anaerobic digestion as a means of treating hatchery waste. It is by far the most popular process used to treat organic wastes in a number of other organic waste industries. The biosolids remaining after the digester process can be sold as a high quality fertiliser. Kumar [49] demonstrated the use of a two stage anaerobic process for effluent from piggeries and abattoirs will produce methane and the nutrients can be also recovered from the waste to grow algae, zooplankton and fish. The treatment system has clear advantages in cost and environmental benefit due to recycling of waste. The benefits also include income from the sale of electricity generated through biogas and fertiliser to produce bioproducts such as algae, zooplankton and fish as livestock feed. Anaerobic digestion involves the degradation and stabilisation of an organic waste under anaerobic conditions by microbial organisms to produce methane and inorganic products [13] as described in the equation below. Organic matter + water ( anaerobes ) → CH 4 + CO 2 + new biomass + NH 3 + H 2 S + heat

This process is commonly used to treat wastewater sludge and organic wastes as it reduces the waste volume, produces valuable products and reduces the emission of methane and CO₂ from land fill sites. The end product of digestion is the nutrient-rich solid which can be used as a fertilizer [50].

There are two basic types of anaerobic digesters;

Batch: Batch digesters are the simplest. The process involves loading the waste into the digester and starting the digestion process. The retention time depends on temperature, pH and other factors. Once the digestion is complete, the residue is removed and another batch started.

The following examples involve treatment of organic waste which may be adaptable to hatchery wastewater with various levels of solids;

Covered lagoons: The organic waste is covered by a pontoon or other floating cover. This digester is suited for manure waste with 2% or less solid content and requires high throughput to provide enough solids for the bacteria to produce gas. It is better for warmer regions, where digester temperatures can be easily maintained.

Since the 1970s, underground anaerobic digesters at various scales of operation to process rural organic wastes have been used in China. In these systems, a cylinder shaped reactor made from concrete and brick with a cement lid are used. Waste is manually fed into the reactor through a port connected to the base of the reactor The heavy lid prevents gas leaks [52] and pressurises the methane produced enabling it be piped to various areas for domestic or commercial use.

This is a process where various organic wastes are mixed for co-digestion. The advantage of co-composting and co-digestion is it achieves a better balance of nutrients and can improve the treatment efficiency [53], particularly for pig and poultry waste at land fill sites. The effect of using various ratios of pig and poultry waste on gas production was studied by Magbanua et al. [54]. The waste was incubated at 35 ± 2 °C for up to 113 d. This process produced high levels of biogas [54]. In another trial best results for methane production were achieved when 3 wastes were used; cattle and poultry waste and cheese whey (w/w on dry weight basis). The digesters initially operated at 40 °C with a 10 day retention time [55]. Clearly hatchery waste has the potential to be included as a material for co-digestion.

Song et al. [56] used two stage anaerobic digestion of sewage sludge. While there is an additional energy requirement required to operate a two stage digester, the advantages of the system is the higher reduction of volatile solids, and increased methane production compared to single stage mesophilic or thermophilic digesters. Likewise the pathogen reduction was similar in the two stage system. More recently Dareioti [57], Sakar et al. [58]; Kara et al. [59]; Parawira et al. [60]; Nishio and Nakashimada [61]; Perez-Elvira et al. [62]; Alatriste-Mondragon et al. [63] and Ke ShuiZhou et al. [64] have reported on the role of and variations to approaches in two stage anaerobic digestion in treating a range of organic wastes. Kumar [49] used two stage anaerobic digestion on piggery effluent. The first stage involved the hydrolytic and acidogenic process and the second stage involved the methanogenic process. The two-stage anaerobic digestion system was configured such that the thermophilic acidification reactors operated with 7 days of hydraulic retention time at 55-70 °C while the mesophilic digesters operated with 22 days of hydraulic retention at ambient temperature. There was a low degree of acidification in the first stage due to the low organic content of the raw piggery effluent which allowed the methanogens to establish and generate methane (>40% methane composition). This was not the desired function of the first-stage reactors, as the majority of methane needs to be generated in the second-stage (methanogenic). Increasing the organic load of the raw piggery effluent was found to be necessary to produce a higher level of volatile fatty acids in the first stage or by adjusting the pH of the effluent by addition of acids.

Kumar [49] reported that increasing digestion temperature from 55-70 °C reduced pathogen survival in an exponential pattern with concomitant die-off at longer hydraulic retention time. The pathogens, Clostridium perfringens and Campylobacter jejuni, survived thermophilic digestion at 50 °C. At the lower thermophilic temperatures, larger numbers of Clostridium perfringens were found in the mesophilic stage. Other strategies to reduce pathogen survival include aeration, free ammonia, biological control (Tetrahymena spp.) and peractic acid. Free ammonia levels reduced the survival of these pathogens, but it is not clear if the solubilisation would also be reduced if free ammonia levels were manipulated artificially. Peracetic acid, as a strong oxidising agent, effectively reduced the number of these pathogens at very low concentrations.

Digesters can be used for any biodegradable waste. However, for biogas production high organic levels in the sludge are required to produce high gas yields from the system. The composition of the waste is a major factor influencing methane production. More gas is produced if the material has high moisture content, a large volume and surface area. The optimal C:N ratio for a microbe is 20–30:1 [65]. The waste contamination level is a key factor affecting anaerobic digestion. If waste has significant levels of physical contaminants such as plastic, glass or metals then pre-treatment will be required. The digesters will not function efficiently if these contaminants are not removed. Often the waste is shredded, minced and mechanically or hydraulically pulped to increase the surface area that is available to microbes in the digesters and increase the speed of digestion. Reactors/digesters should be designed according to the waste characteristics to achieve effective digestion results. Some of the reactors include;

Reactor with membrane filtration: Membrane filtration improves the stability of the digestion process and reduces hydraulic retention time for wastewater treatment. It also retains all micro-organisms to improve the efficiency of the process [66,67].

Sabry [81] concluded that UASB reactor could treat any type of liquid organic waste sludge. It is envisaged that the liquid portion of the hatchery waste could be used as a waste stream in these reactors. The suitability of the UASB process for the pre-treatment of a liquid part of hen manure has been reported. The OLR is 11-12 g COD/L/day and HRT is 1-2 days with an efficiency of 70-75% on the basis of total COD reduction [82].

If the wastewater contains high organic materials with high nutrients such as abattoir or hatchery wastewater, pre-treatment is required before discharge to the ponds. Cavitated Air Flotation system can reduce content of fat, oil and grease and suspended solids through polymeric chemical coagulation and flocculation.

Growing algae in wastewater ponds such as abattoir or hatchery wastewater could be an option if lagoons are used at hatcheries to dispose wastewater. The wastewater is managed through a series of ponds to produce algae, zooplankton, ornamental fish and cleaner water that is suitable for irrigation. (Note that the ornamental fish industry is a mutli-billion dollar industry worldwide with high demand for various fish species). However, good management and design of ponds are required to optimise ornamental fish production. Fresh water molluscs are potential species to introduce into the ponds to filter the waste water and improve algal and zooplankton growth. The zooplankton and micro-algae are grazed by the fish. Management of nutrients in ponds (particularly P levels) is critical if pond production is to be optimised [49].

Temperature plays an important role in bacterial digestion. The optimal function temperature for mesophilic bacteria are 32.2-43.3 °C and 48.9-60 °C for thermophilic bacteria. Thermophilic digestion eliminates more pathogenic bacteria but requires higher energy costs to achieve the higher temperatures required and may be less stable. Digestion slows down or stops completely when the temperature is below 15.6 °C. Maximum conversion occurs at about 35 °C in conventional mesophilic digesters. The amount of methane produced decreases with decreasing temperature [52,102]. Maintaining stable temperatures in the reactor is very important. Fluctuation of temperature inside the digester can cause system failure [52].

Digestion will proceed well with a C:N ratio between 15:1 and 30:1 (optimum is 20:1). If the system is over loaded with organic waste, adding low nitrogen content and high carbon materials such as crop residues or leaves can improve digestion performance [52]. High protein waste will produce high levels of ammonia nitrogen during anaerobic digestion and result in an unstable digestion process, reduce biogas production and produce ammonium toxicity [103-106]. Lipids in the waste could form floating materials and accumulate long-chain fatty acids in anaerobic digestion [27,107,108].

Trace nutrients are known to influence reactor performance. Whey powder supplemented with nitrogen and phosphorus was found to be limited by either Ni, Fe or Co, or a combination of those elements. After the addition of these elements to a reactor, COD removal efficiencies increased and the level of volatile organic acids decreased [91]. The effects of ionic chromium, cadmium, lead, copper, zinc, and nickel on the methanogenic UASB have been examined [109]. The effects noted for metals depend on types of metal, zones of sludge, types of VFA and HRT. The relative toxicity of the metals to total VFA degradation was Cu>Cr>Cd=Zn>Ni>Pb for both bed and blanket sludges. However, different levels of toxicity were found for individual VFAs and sludges. For the degradation of total VFA, the copper toxicity resistance of blanket sludge was lower than that of the bed sludge [109].

A uniform loading of manure with 6–10% solids on a daily basis generally works well with the load's retention time in the digester ranging from 15–30 days [52]. An issue with digesters is that blockages can be caused by using waste resources that have a variable solid content. In the case of hatchery waste the content of solids can be variable depending on whether most of the waste comprises mainly non fertile eggs that are removed from the incubators during incubation or if the waste is made up mainly of dead embryos after hatch.

The effluent in the digester needs to be mixed regularly to prevent settling and to maintain contact between the bacteria and the manure. The mixing action also prevents the formation of scum and facilitates release of the biogas [52]. In the case of livestock effluent it sometimes contains fibrous materials which can cause blockage in digesters.

Regular monitoring, maintain a constant required temperature of the digester is important if the system is to run smoothly. Failure to properly manage the digester can result in poor gas production and some systems take months to get back into operation if there is a breakdown [52]. Quality control needs to be implanted to manage the waste resources being utlised in the digester as well the biogas and other products being produced. Records need to be maintained on biogas and other products produced and the effluent that has been fed into the system [52].

Biogas is primarily composed of methane and carbon dioxide, with traces of ammonia and hydrogen sulphide. Caution should be taken as methane is highly explosive when mixed with air. In addition, because digester gas is heavier than air, it displaces oxygen near the ground, and if hydrogen sulfide is still present, the gas can act as a deadly poison. It is critical that digester systems be designed with adequate venting to avoid these dangerous situations. Exposure to any of these gases may result in ill-health or death, and levels in the biogas may vary widely and cyclically. Carbon dioxide, ammonia and hydrogen sulphide are all toxic gases, and are subject to the regulations as substances hazardous to health [52].

Because biogas is not used at exactly the same rate at which it is produced, it must be stored somewhere. The gas must be efficiently transported from digester to storage tank. Because of the high pressure and low temperature required, it is impractical to liquefy methane for use as a liquid fuel. Instead, the gas can be collected and stored for a period of time until it can be used. The most common means of collecting and storing the gas produced by a digester is with a floating cover-a weighted pontoon that floats on the liquid surface of a collection/storage basin. Skirt plates on the sides of the pontoon extend down into the liquid, thereby creating a seal and preventing the gas from coming into contact with the open atmosphere. High-pressure storage is also possible, but is both more expensive and more dangerous and should be pursued only with the help of a qualified engineer [52].