Using digestate – the liquid overflow from a digester

Using digestate – the liquid overflow from a digester.

“It has a high nutrient value – the exact makeup of fertlisier will vary depending on the nature of the feedstock. However it has a high nutrient value, generally around

4 to 5 kg / 1000 litres of nitrogen

1-2 kg / 1000 litres of phosphate

1-2 kg / 1000 litres of potash (potassium carbonate)

2 kg / 1000 litres of magnesium” from Cranfield Dr Frederic Couklon and Dr Raffaella Villa. www.cranfield.ac.uk

The liquid overflow from an anaerobic digester is called digestate. It flows out at about the same rate that water is flowed into the digester. Some solids have been removed by settling ( they were too heavy and are now at the bottom of the tank), by being eaten by bacteria and tturned into gas, and by forming a scum layer on top of the liquid. The remaining dissolved solids are still retained in the liquid that flows out of the digester.

An example is shown below:

 

Sample site 1 Sample site 2
Acceptable Sewerage Values BGD inlet BGD outlet ABR outlet reedbed outlet COD removal across BGD BGD inlet BGD outlet BGD outlet reedbed outlet polishing pond outlet COD removal across BGD
1 2 3 4 5 6 6b Oct 07 7 8
pH Value 5.5 – 12 at 25 C 6.44 6.61 6.85 7.07 6.35 7.11 7 6.56 7.74
Conductivity 500 at 25 C in ms/m 190 168 150 134 284 130 102 72.7 135
Sodium Absorption Ration 7.93 2.68 2.38 2.31 4.65 8.23 8.46 8.01
Chemical Oxygen Demand 5000mg/l 12870 196 275 69 98.5% 21515 491 233 835 157 98%
Total Dissolved Solids@105 C 4000mg/l 12315 961 659 582 13682 974 711 625
Total Dissolved Solids@180 C 11177 849 651 539 10431 772 591 530
Total Inorganic Solids@550 C 3228 491 457 366 1422 332 254 344
Suspended Solids@105 C 1000mg/l 11040 120 68 44 11500 312 680 76
Calcium (Ca) 94 111 108 97       < 1         < 1         < 1        < 1
Magnesium (Mg) 18 12 11 11 33 11 7 12
Sodium (Na) 1000mg/l 118 111 97 90 124 157 112 105 129
Potassium (K) 31 25 20 19 86 25 7 26
Chloride (Cl) 1500mg/l 130 134 134 125 193 164 121 175
Phosphorus (P) 25mg/l 13.2 11.3 7.39 5.79 10.2 7.05 3.36 5.46
Total Kjeldahl Nitrogen(N) 925 111 77 59 750 113 43 85
Ammonia (NH4) 97 26 19 15 280 22 9.2 22
Nitrite (NO2)                     ND 0.41 0.33 0.33              ND 0.77 0.2 1.39
Nitrate (NO3)                     ND 0.04 0.4 0.4              ND 0.77 0.5 0.76
Presumptive Coliform Count MPN (Faecal) 1600+ 1600 1600 1600 1600 1600 1600 1600
Confirmed Coliform Count MPN (E. coli Type 1 1600 1600 1600 1600 1600 1600 1600 1600