Can One Size Settlement Lagoon Fit all Situations

T: 0113 2524786 Email Melvyn.Rutter@btinternet.com M: 07884 253827

Settlement Lagoon Figures

The question for settlement lagoon is, can one size fit all. This would seem unlikely. However the following calculations are for three scenarios for a farmyard area of 500m2. The figures in the lists below are in millimetres. Although the terms have been related into inches, as this seems to be the common term for rainfall in the UK. The calculation used has been 25mm per inch.

The figure in A, of 12.5 is the number of cubic metres of rainfall over 500m2 of farm yard.

This first scenario poses a regular amount of rainfall over 2 weeks where :-

A = 1 inch rainfall would produce 12.5m3 of farm yard runoff flowing to the lagoon.

( the figure in A of 50, represents the lagoon being half full )

B = Daily volume filtered in the reedbeds

C = The volume in cubic metres in the lagoon

The total volume of the proposed lagoon is 100m3.

A	50	12.5	12.5	12.5	12.5	12.5			12.5	12.5	12.5	12.5	12.5
B	8	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	8
C	43	47.5	52.5	57.5	62.5	67.5	60	53	57.5	62.5	67.5	72.5	77.5	70

Over a two week wet period the levels never completely fill the settlement lagoon. However this is far too simple an example, and nature has a way of being more random and dangerously far more irregular.

This second scenario poses a stratified random level of rainfall. In this proposed 4 week period there are :-

3 days with 2 inches of rainfall

10 days of 1 inch of rainfall

9 days of ½ inch of rainfall

5 days of no rainfall

A

12.5

7.5

42.5

47.5

68.5

65.5

75.5

80.5

85.5

90.5

0	0	0	6	13	25	12.5	6	6	6	12.5	12.5	6	0
7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5
87	79	72	70	75	73.5	78.5	77	75.5	74	79	84	82.5	42.5

A = 1 inch rainfall would produce 12.5m3 farm yard runoff flowing to the lagoon. 2 inches would produce 25m3 of farm yard runoff flowing to the lagoon. 6m3 represents half an inch of rainfall.

( the figure in A of 50, represents the lagoon being half full )

B = Daily volume filtered in the reedbeds

C = The volume in cubic metres in the lagoon

The total volume of the proposed lagoon is 100m3

There is a danger of lagoon levels exceeding capacity in days 13 and 14, but otherwise levels remain constant. Once more, this scenario begins with the lagoon being half full.

This final scenario poses a stratified random level of rainfall. In this proposed 4 week period there are :-

7 days with 2 inches of rainfall

6 days of 1 inch of rainfall

9 days of ½ inch of rainfall

5 days of no rainfall

On this scenario the lagoon begins as being empty.

A	0	25	25	25	6	6	6	0	25	25	12.5	12.5	13	6
B	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5
C	0	18	35	52.5	51	49.5	48	40.5	58	75.5	80.5	85.5	91	89

0	0	0	6	25	25	13	6	6	6	12.5	12.5	6
7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5	7.5
82	74	67	65	82.5	81	86	84.5	83	81.5	86.5	91.5	90

A = 1 inch rainfall would produce 12.5m3 farm yard runoff flowing to the lagoon. 2 inches would produce 25m3 of farm yard runoff flowing to the lagoon. 6m3 represents half an inch of rainfall.

( the first figure in A, represents the lagoon being empty )

B = Daily volume filtered in the reedbeds

C = The volume in cubic metres in the lagoon

The total volume of the proposed lagoon is 100m3

Once more the lagoon levels are not exceeded. Calculations carrying this scenario on indefinitely show the lagoon levels could exceed capacity after 30 days. However, if the flow rates were increased to 10m3 per day after day 28 of this proposed heavy rain situation, then lagoon levels will exceed capacity for 5 days in days 37 to 42, over a 54 day period. Indeed, if after 20 days the daily flow rate through the reedbeds were increased to 10m per day, lagoon levels would not be exceeded until day s 63 to 68.

By day 63 in this scenario there would have been more than 307 inches of rainfall ( 770mm, with 2.5mm = 1 inch ).

If the settlement lagoon was 10m x 6m x 2m deep, and therefore holding 120,000 litres, or 120m3, then we could be confident for 99% of the time that levels would not be exceeded.

Sewage works operate Combined Sewer Overflows ( CSO’s ) systems, where in heavy rainfall, untreated industrial and domestic waste water is allowed to flow directly to streams and rivers. One water company has 3,600 CSO’s. In the scenarios listed above, all sewage works would have operated their CSO’s on numerous occasions.

Therefore we can be confident in the argument that one size settlement lagoon could fit all situations for farmyard areas of 500m2. For farmyard areas more greater than that listed above, the settlement lagoon could be built to have a larger capacity, without the need for a greater area of reedbeds. Indeed, if the lagoon size was doubled, then we could have rainfall of 614 inches in 63 days, without the need for a larger reedbed.

With rainfall of 614 inches in 63 days, our first priority would be to get into a boat. Concerns about reedbeds and settlement lagoons would not be troubling us.

Clearly, care has to be taken with calculations, and the setting up of reedbeds, lagoon and pump rates. However, a level of certainty can be afforded the concept of having a system to effectively filter farm yard runoff in a 100m3 settlement tank, which leads to 100m2 vertical flow reedbed, which then discharges to a living soakaway, so that there is no direct discharge to a water course.

Return to the Top of the Page

Contact :- Melvyn.Rutter@btinternet.com

T: 0113 2524786 M: 07884 253827