3.6 What are the output characteristics after aerobic digestion?
The output characteristics after aerobic digestion vary on the operational conditions.
If an aerobic digester is operated at optimal temperatures and solids retention time, it provides solids reduction similar to that of an anaerobic digestion system. The process can reach a volatile suspended solids (VSS) reduction ranging from 40 to 50%. If temperatures are low (<15°C), VSS reductions may fall to 35 to 40% unless very long retention times are provided.
The degree of pathogen reduction is similar to solids reduction in that little pathogen reduction can be expected at temperatures less than 10°C, while significant reduction may be achieved at temperatures greater than 20°C.
The table below outlines the characteristics of effluent/supernatant resulting from aerobic digestion systems. The values presented by the Water Environment Federation (WEF) indicate typical ranges of effluent parameters achieved through the aerobic digestion of sewage sludge (SS). Meanwhile, the data from UPM reflects specific results of effluent characterization from faecal sludge (FS) treated by aerobic digestion in the Rohingya refugee camp in Cox’s Bazar, Bangladesh, in 2019.
| Parameter | SS-based Effluent (WEF) | FS-based Effluent (UPM) | Unit |
|---|---|---|---|
| TSS | 46 – 11,500 | 375 | mg/L |
| BOD5 | 9 – 1,700 | 912 | mg/L |
| COD | 288 – 8,140 | 3,450 | mg/L |
| TKN | 10 – 400 | 924 | mg/L |
| TP | 19 – 241 | 17 | mg/L |
| pH | 5.9 – 7.7 | 8.4 | – |
| Faecal Coliform | – | 2,000 | cfu/100mL |
| Helminths Eggs | – | Nil | Eggs/L |
The parameter values of the effluent sample from the aerobic digester in the Rohingya refugee camp fall mostly within the ranges indicated by the WEF literature source. Only the value for total nitrogen is increased, indicating a high concentration of nitrogen compounds in the sample. Additional steps for nitrogen removal may be required.
The solid fraction that settles in the digester following the aeration phase has different characteristics and requirements for post-treatment compared to the effluent. In the example from the refugee camp in Cox’s Bazar, samples of these solids/biosolids were taken from the aerobic digester and after the post-treatment (drying and lime treatment). The results are presented in the following table.
| Parameter | Biosolids from the aerobic digester | Biosolids after lime treatment | Unit |
|---|---|---|---|
| pH | 8.3 | 14 | |
| TS | 3.5 | 39 | % |
| VS | 73 | 29 | %TS |
| TOC | – | 36,300 | mg/kg |
| TKN | – | 16,800 | mg/kg |
| TP | – | 2,293 | mg/kg |
| Faecal Coliform | – | Nil | |
| Helminths | – | Nil |
The extent to which the biosolids can be utilized or if they require further treatment will depend on the intended use and on compliance with regulations governing biosolid management.
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