Batch storage of manures

Batch storage of manures is important because very large numbers of bacteria can fit into very small spaces and so new additions of manure to an FYM heap or slurry tank have the potential of substantially increasing the bacterial load of the waste.  For example, Figure 1 below shows how generic E. coli numbers fall over time in cattle FYM. 

Figure 1  Declines of E. coli in beef cattle FYM (reproduced with permission Hutchison et al., 2005)

Figure 1 tries to show what happens to the numbers of E. coli in an farmyard manure heap when continual additions of waste are added.  The black line shows how the numbers of E. coli fall over time.  The red, green and yellow lines are the same graph, but represent bacteria added as more manure is added to the heap.  In more detail, on day zero, let’s say we start off with 1 tonne of manure in the store.  Since there are 100,000 E. coli contained within each gram of that waste, we have 100,000 E. coli x 1000 (the number of grammes in a kilo) x 1000 (the number of kilos in a tonne) = 100,000,000,000 E. coli.  For convenience, we will write 100,000,000,000 as 112 – a number 1 with 11 zeros after is 12 digits in total; so 112.  That’s a lot of bacteria, but they soon begin to die off.  After 10 days, there are only 80,000 per gram of waste.  Repeating the same calculation above, that’s 811 (a number 8 with 10 zeros after it gives us 11 digits in total).  If, on the 10th day, another 1 tonne of waste is added to the store then we have, on average, (811 + 112)/2 bacteria = 911 bacteria.  As before these will die off, but the original E. coli in the waste dies off following the black decline line; and the newly added E. coli following the red line.  At 40 days, there are very few (about 6,000 per gram) of the original E. coli left; and the E coli added after 10 days have fallen to about 20,000 per gram.  Thus the average number of E coli is 13,000 per gram in the two tonnes of waste we have. Doing the same calculation as above that’s 13,000 x 1000 x 1000 x 2 = 2.610 E. coli (which is still a lot of E. coli even though its only about 2.5% of the number we started with). Table 1 below summarises what happens to the numbers of E. coli if three additions, each of one tonne of waste are made to a manure heap at days 10, 40 and 60 and the material is spread to land on day 70.

Table 1 A summary of E. coli numbers in cattle manure waste if additions of fresh material are added to the manure heap

 

Numbers of E coli in each batch of added waste

Day

Original heap

Addition 1

Addition 2

Addition3

Total E. coli per gram of waste

Total mass of waste (tonnes)

Total E. coli in the heap

 

 

 

 

 

 

 

 

0

100,000

 -

 -

 -

100,000

1

112

10

80,000

100,000

 -

 -

90,000

2

1.812

40

6,000

80,000

100,000

 -

62,000

3

1.912

60

0

6,000

80,000

100,000

46,500

4

1.912

70

0

0

6,000

80,000

21,500

4

8.611

 

As you can see from table 1, at day 70 there are 8.610 E. coli in total in the waste heap (or if you prefer 86% of the number we started off with in the heap at day zero).  Despite some of the material being stored for 70 days – long enough for all the E. coli in that to have died out in that material completely; we still have enormous numbers of E. coli in the material being spread to land.  The E. coli mostly come from the most recent addition of waste to the store. The effect is a consequence of the fact that very large numbers of E. coli can be present in relatively small quantities of wastes.  Furthermore, the numbers of E. coli used in the example above are a bit on the low side (to keep the numbers of zeros in the discussion and table manageable) and so in a typical real life situation, the effect is even more pronounced.

Finally, the above explanation provides a simplified view of bacterial decline in manures.  A separate problem caused by continuous additions of manure to a store is that the waste contains nutrients as well as additional pathogens.  The nutrients can be consumed by bacteria (Pötter et al., 2001) and thus it is widely speculated that the practice of continuous addition extends pathogen survival in manure stores.  The actual effects of continuous nutrient supply to manure heaps are however not well described. There are not yet any substantial publications that fully report the implications of the practice.

References

Hutchison M. L., Walters L. D., Avery, S. M. and Moore A. 2005. Decline of zoonotic agents in livestock waste and bedding heaps.  J. Appl. Microbiol. 99, 354-362.

Pötter, M., Oppermann-Sanio, F. B., and Steinbüchel, A. 2001. Cultivation of Bacteria Producing Polyamino Acids with Liquid Manure as Carbon and Nitrogen Source. Appl Environ Microbiol. 67, 617–622.