Biosolids reduction by extending SRT – Economic aspect

The amount of biosolids produced can be reduced by extending solids retention time (SRT) as discussed with Fig. 1 here.For example, according to the graph, biosolids yield decreases from 0.33 g MLSS/g COD to 0.28 g MLSS/g COD by extending SRT from 20 days to 30 days based on ASM#1 model. One thing must be noticed is that biosolids reduction is becoming increasingly harder as SRT increases due to the accumulation of non-biodegradable materials in the biosolids. Therefore, the required aeration tank volume increases more than proportionally predicted based on the data from initial biosolids reduction.

   Significant capital cost increases are inevitable to realize the biosolids reduction due to the following reasons.

  • ¬† If same MLSS as MBR are employed, e.g. 8-12 g/L, bioreactor volume must increase significantly to obtain sufficient SRT. The large aeration tank volume and additional aeration systems are perhaps the bottleneck that does not allow economical feasibility in ordinary circumstances. In addition to the capital cost constraint, under the high SRT condition, oxygen uptake rate (OUR) can be much lower than ordinary MBR due to the slow endogenous respiration and the accumulation of biologically inactive non-biodegradable materials. Therefore, the aeration for mixing can exceed the aeration required to supply oxygen at high SRT, which causes waste of aeration energy. This suggests that there is a maximum aeration tank volume with which air demands for mixing and oxygenation match each other.¬†
  • ¬† If high MLSS than in MBR are employed, e.g. 30-40 g MLSS/L, membrane flux cannot be maintained as in ordinary MBR running at 8-12 g MLSS/L. In excess sludge thickening process to produce a sludge with 30-40 mg MLSS/L, fluxes are maintained less than a half of the ordinary flux. Therefore, capital costs for membrane systems are more than double the ordinary MBR.

   Apart from the capital costs, even operational costs are not favorable for the biosolids reduction based on high SRT due to the following.

  • ¬† As calculated in the attached calculator, 1 g of biosolids need 1.37 g O2 to be decomposed assuming all the nitrate ions produced from the process are recycled for oxygen credits and all the inorganic materials in biosolids are removed as dissolved solids simultaneously with the organic materials.
  • ¬† While the specific power consumption to dissolve unit amount of oxygen is calculated here at 1.22 kWh/kg O2 for ordinary MBR with 4.5 m depth, it can be assumed a double at high MLSS conditions, e.g. 30-40 g MLSS/L. This is a conservative assumption because OTE can decrease to well less than 1/4 of the original OTE when MLSS increases from 10 g/L to 40 g/L according to the equations in Fig. 1 here.
  • ¬† Considering the specific power consumption of 1.22 kWh/kg O2, 2.56 kWh is required to dissolve 1 kg O2, which can convert to 3.51 kWh per 1 kg MLSS reduction.
  • ¬†¬† Finally, around $351 of power costs are required to reduced 1 ton of biosolids as dry mass excluding other operating costs and capital costs (=3.51 kWh/kg x 0.1 $/kWh x 1,000 kg/ton).

The biggest uncertainty in the calculation is the oxygen transfer efficiency at high MLSS. Nonetheless, given the conservative assumption made in the calculation, it is clear that the operating costs are quite significant. The variables in the calculator attached can be modified.

   Meanwhile, the high MLSS applications of immersed membranes are much more common for excess sludge thickening, where only the small amount of excess sludge from biological process is filtered by membranes to reduce the volume. Design fluxes are typically less than a half of the ordinary MBR due to the high MLSS employed. When waste activated sludge (WAS) was thickened from 3-8 g/L to 45 g/L using immersed hollow fiber membranes (ZW500d), flux was gradually reduced from 7.75 LMH to 2.5 LMH during the course of the thickening process to mitigate membrane fouling (Natvik, 2009). In other example, target MLSS was set at 40 g/L with similar membranes (Cantor, 2000)

 

© Seong Hoon Yoon