Dissolved air flotation (DAF) is widely applied to treat wastewater containing high pollution load that mainly is built up by solids and fat. Depending on the required efficiency, DAF without chemicals is applied to meet minimum effluent requirements. To reach a higher efficiency, physical-chemical primary treatment or DAF with chemicals is applied. In some cases, subsequent treatment may be necessary. Nevertheless, this so-called “classic” configuration generates sludge with high fat content that usually ends up disposed while this sludge has a high energy potential and can be valorized into valuable products.
Clients around the world are looking for ways to lower their environmental footprint by reducing the use of chemicals and disposal of sludge. Based on these demanding requirements, Nijhuis Industries developed a separation solution: AECO-FAT. This solution effectively treates wastewater and delivers a high-quality recovered product.
Profitting from Waste
The first step of the solution involves a chemical-free DAF system (see Fig. 1, page 13) which removes approximately 50% of incoming chemical oxygen demand (COD). Depending on the non-emulsified fat fraction, approximately 50% to 80% of incoming fat can be separated. Effluent water from this chemical-free DAF contains less pollution load and can be further treated in the existing wastewater treatment system to meet effluent discharge requirements. As such, consumption and sludge production is reduced in subsequent treatment.
The sludge from the chemical-free DAF is further treated in a non-reactive method in the disconnector tank to initially separate the bounds between solid and fat fractions. This pre-treated sludge then is separated into solids, light-liquid and heavy-liquid fractions. The solids fraction contains high dry solids content, which can be used for land application as a soil amendment when applicable. Heavy liquid or centrate water is sent back to the buffer tank or secondary treatment. High-quality light liquid or the recovered fat is collected in an insulated tank to be further applied as an energy source. This biofuel powers the boiler/burner at the factory or can be sold externally.
A pilot installation was operated in the Netherlands at GPS, a poultry slaughterhouse, in Nunspeet, and at a rendering facility in Wijster, also in the Netherlands. The system resulted in a fat recovery rate of 85% or higher and a recovered fat purity of 95% or higher.
Low fatty acids and sulphur contents in the recovered fat were achieved at the poultry slaughterhouse, while the recovered fat at the rendering facility contained somewhat higher fatty acids and sulphur content. When considering suitability of application for biodiesel production from vegetable and animal fat, low fatty acids and sulphur contents are important requirements, as is a next to low concentration of water in oil. This has shown that different types of sludge will influence the recovered fat quality. In these cases, recovered fat from treated fresh wastewater was suitable for biodiesel production.
Top to Bottom Optimization
The poultry slaughterhouse processes 6,250 birds per hour, which corresponds to 39,1758 gal per day (gpd) of generated wastewater. To perform a good working and high-efficiency primary treatment, ferric chloride (FeCl3) has been consumed as coagulant and anionic polyelectrolyte as flocculant, without consumption of neutralizer such as sodium hydroxide (NaOH). With the separation solution, the chemicals consumption is decreased by 50% due to the efficiency of the first step where the load was reduced by 50% (see Fig. 1, page 13). Sludge disposal cost also was reduced as a result of less total sludge produced, due in part to the recovery of fat. Furthermore, in combination with high dry solids content in the cake, in total there was less sludge volume to discharge.
At least 2,600 MW of energy will be produced from the recovered fat annually, which corresponds to savings of €23.00 for each MW of energy by using natural gas. Next to the investment for the separation technology, additional investment for suitable burner may be required to apply the recovered fat for thermal energy at the factory.
Next to the potential revenue, the separation solution also offers sustainability by reducing greenhouse gas emissions by at least 60%, including the emission from the system to produce the recovered fat, which already is relatively low. When translated to carbon footprint, the maximum allowable total greenhouse gas emission in combination with the applicable recovered fat is 31 grams CO2 eq/MJ .
This case illustrates that a payback period of two years or less can be achieved for the poultry slaughterhouse with a wastewater flow of 1,500 cu meters per day or higher. This also is the case for other food and meat processing industries containing a high amount of fat in wastewater.
For instance, a bacon-processing factory in Ireland with the full-scale separation installation produces 158,053 gal of wastewater per day, resulting in 7,925 gal of fat-rich sludge per day, which is further separated to recover 1,585 gal of almost pure fat per day. Half of this nearly pure fat is used as a thermal energy source in the factory and the rest is sold for biodiesel production, allowing for a payback period of 18 to 24 months.
Another modern poultry slaughterhouse in the Netherlands operates at a typical slaughtering capacity of 13,000 birds per hour. With its high-end slaughtering machines, the water consumption is reduced. Unlike common poultry slaughterhouse practices in Western Europe that typically consume 11 liters of water per bird, a modern poultry slaughterhouse consumes 5 to 7 liters of water per bird. On the wastewater side, the different waste streams were optimized and the highly concentrated fat was collected, which was an ideal condition for further treatment with the AECO-FAT solution.
This slaughterhouse practice leads to a lower wastewater volume generation, resulting in a lower treatment capacity requirement compared to a traditional poultry slaughterhouse. Accordingly, one is able to select a smaller solution with a lower investment cost without compromising the treatment efficiency and quality. Despite the lower wastewater volume, the fat concentration still is the same, as is the amount of recovered fat products.
With this situation, a shorter payback period will be achieved. The recovered fat quality from modern poultry slaughterhouse practice is expected to be higher with lower impurities compared to the traditional poultry slaughterhouse.
Other possible applications for the recovered products are biodiesel production, co-substrate for anaerobic digestion, animal food and cosmetics industries.