Introduction:
This study evaluated the effectiveness of the NovaWAVE microwave digestion system and entailed the digestion of sludges and waste water for TKN determination. The results are compared to the standard digestion on a DigiPREP HT block. 

Supplies and Reagents:

1) NovaWAVE SA Model* 2) Quartz 75 ml Vessels* 3) Teflon® Caps and Safety Pressure Release Caps (pre-set release pressure to 500 psi)* 4) PlasmaPURE H2SO4 (98%)* 5) PlasmaPURE H2O2 (30%)* 6) PlasmaPURE H2SO4 (0.1 N and 0.001 N) 7) Boric acid (2%) 8) NaOH (12.5 M) 9) Methyl Orange indicator 10) DigiPREP 400 Automatic Steam Distillation Apparatus* 11) HT 250 ml vessels * 12) 500 ml Erlenmeyer Flask 13) DigiPREP HT* * Manufactured by SCP SCIENCE

Sample Type:

• Sludge – 6% solids (0.5-1 g)
• Waste water – 5 ml

Sample Preparation Procedure:
A sample of 1 g of sludge was put first in a 75 ml quartz vessel followed by the addition of 5 ml of H₂SO₄ and 10 ml of H₂O₂. The solution was left to react at room
temperature for 10-15 minutes as some frothing might occur. The vessel was then placed in the microwave and digested following the temperature profile below. At the end of the digestion, the vessel was vented and transferred to a HT 250 ml vessel containing 85 ml of water for distillation.

Temperature profile for TKN digestion - sludge (MW)

Left: sludge before MW digestion. Right: sludge after MW digestion

Preparation of samples – sludge (HT)

A sample of 1 g of sludge was put first in a HT 250 ml vessel followed by the addition of 15 ml of H₂SO₄ and 0.02 g of CuSO₄. The vessel was then placed on the HT block and digested following the temperature profile below. At the end of the digestion, 85 ml of water was added to the vessel.

Temperature profile for TKN digestion - sludge (HT)

Left: sludge before HT digestion. Right: sludge after HT digestion

Preparation of samples – waste water (MW)
Sulfuric acid (5 ml) was put first in a 75 ml quartz vessel followed by the addition of 5 ml of waste water and 5 ml of H₂O₂. The vessel was then placed in the microwave and digested following the temperature profile below. At the end of the digestion, the vessel was vented and transferred to a HT 250 ml vessel containing 85 ml of water for distillation.

Temperature profile for TKN digestion – waste water (MW)

Preparation of samples – waste water (HT)
Sulfuric acid (15 ml) was put first in HT 250 ml vessel followed by the addition of 5 ml of waste water and 0.02 g of CuSO₄. The vessel was then placed on the HT block and digested following the temperature profile below. At the end of the digestion, 85 ml of water was added to the vessel.

Temperature profile for TKN digestion - waste water (HT)

Distillation
NaOH (75 ml) was added to the HT 250 ml vessel and a solution of 2% boric acid was placed in the receiving distillation erlenmeyer. The distillation was performed following the profile noted below. Approximately 100-150 ml of distillate should be collected.

Distillation profile TKN digestion

Titration
The distillate is titrated with 0.1 N H₂SO₄ for the sludge and 0.001 N for waste water using 3 drops of methyl orange indicator until the solution turns pink.

Recovery for sludge

(1) AVG of 4 digestions

(2) Blank substracted

Recovery for waste water

(1) AVG of 4 digestions

(2) Blank substracted