HOW THE LAWS OF PHYSICS ARE BOTH THE CHALLENGE AND THE SOLUTION FOR LONG-TERM, SMART MANURE STORAGE IN A FLEXXOBAG
On average, a cow produces about 100 kg of urine and manure per day . The urine and manure of a cow (or pig) can be used for natural fertilisation of the land. With an average of about 80 cows per (dairy) farm , solutions are needed for the storage of these large quantities of urine and manure. The Flexxobag is a sustainable and cost-efficient solution for the storage of liquids, mainly manure from cattle, in large volumes up to 7000 m³. If necessary, the manure can be pumped from the stable cellar into the storage bag. As time passes, gravity pulls the solid and heavy particles to the bottom of the bag. This makes it a challenge to pump out the contents, if the farmer decides to use it for his land. This challenge can be solved by using mixers. The mixers set the manure in motion and lift the solid particles.
What can we learn from the sinking of these particles and how can we use that information to improve our products and service? Engineers Maarten Steggink and Martin Nieuwmeijer have not only come up with smart solutions for manure storage, but have also conducted research to find out more about it. You can read the results (among others) in this blog.
Objectives for mixing are: 1. Setting the manure in motion; 2. Achieving a speed that erodes the sunken solid; 3. Maintaining a speed that keeps the contents in motion.
In order to get the best mixing results, the correct configuration and set-up of the mixers is required. Since visual determination of the mixing quality is not possible due to the size, depth and opacity of the liquid, CFD simulations are the best way to advise on mixing. To simulate the behaviour of particles in moving fluids, we looked to nature. In the field of hydrology, much effort has been put into studying the functioning of rivers. One phenomenon found in rivers is the erosion and transport of sediments from upstream to downstream locations in the river’s course. The two main characteristics that determine this phenomenon are the water velocity in the river and the particle size of the eroded material. A minimum velocity is required for a certain size of the particles being eroded/transported. The work of the Swedish geographer Filip Hjusltröm links these two conditions . From Hjulström’s work we can conclude that we have two goals with manure mixing: 1. Maximising the flow rate of the manure in the bag; 2. Maximising the area that reaches a flow velocity of more than 0.5 m/s.
Since manure does not behave in the same way as water, we first studied its properties and implemented them in our simulation model. The main differences of interest in these cases are the density and viscosity of the manure.  From these simulations in Comsol Multiphysics, it can be concluded that a conventional mixer arrangement, for example in the middle or concentric, does not give the best results.
To illustrate, we show two configurations that are often used in the “conventional” market.
Figure 1 above shows a 1000 m³ manure bag with an 11 kW mixer in the middle. This configuration has an average speed of ‘only’ 0.34 m/s and the total mixed area with speed >0.5 m/s is ‘only’ 27%.
Based on our optimisations, we concluded that (with the same conditions like bag size, type of mixer etc.) an improved mixer position and orientation could increase the average speed to 0.53 m/s (+56%) and the mixed area to 51% (+89%). A configuration with two mixers with optimised position and orientation increases the average speed to 0.97 m/s and the sufficiently mixed area to 79%.
Figure 2 shows a 7000 m³ manure bag with cow manure and contains 5 mixers of 11 kW. Our simulation shows that this conventional configuration has an average speed of 0.7 m/s and a mixed area reaching a speed >0.5 m/s of 74%. This configuration requires 55 kW and a high investment in mixers and auxiliary equipment.
In our improved setup, also used in a 7000 m³ manure bag and 11 kW mixers, we learned through our simulation optimisation that significantly better results can be achieved with only 3 mixers. By changing the positioning and geometry, we can achieve an average speed of 1.01 m/s and a mixing area where the speed >0.5 m/s is 87%. This results in a much better mixing performance with 40% less power consumption and 40% less investment.
Where gravity is the challenge to fully empty a manure bag, we find our solution in applying the knowledge we have gained about the behaviour of manure particles. Based on this, we give the right advice on mixer configurations in a Flexxobag.
Although CFD is often called ‘Colors For Directors’, these Computational Fluid Dynamics calculations are very useful in optimising mixing in large manure bags.
By: Maarten Steggink, BSc Applied Physics; Martin Nieuwmeijer, MSc Chemical Engineering; Laura Nieuwmeijer, BSc Industrial Engineering & Management
 Biologische Koeien. (2014, december 2). Extracted from TU Delft Open Research.
 Feiten en cijfers Nederlandse veehouderij. (2019, january 15). Extracted from Melkveebedrijf
 The Hjulström Curve of River Erosion, Transportation and Deposition. (2019, july 7).
 Physical properties of Dairy Manure Pre- and Post-Anaerobic Digestion. Applied Sciences. (2019, July 3) Wang c.s.