Solids reduction and sludge stability

The total solids flux of the influent and effluent are shown for each case in Figure 5-5 and solids data is listed in Table 5-4. For the THP digestion case the influent flux is to the THP+AD combination. Conventional digestion will process 60463kgTSS/d whereTHP digestion is able to process 153855kgTSS/d in the same digester tank volume. The THP digestion case processes over 2.5 times more sludge and would allow over 90 000kgTSS/d additional sludge to be imported from surrounding WWTW’s for beneficiation. Comparing the feed and effluent streams for each case it can be seen how of the biodegradable material was reduced by the AD process, thereby reducing the overall solids flux required for disposal. The change in biodegradable material was converted to biogas and soluble AD products via the stoichiometry given in Section 4.5.3 and 4.5.4. However, some solids were formed in the AD from the precipitation of struvite as discussed in Section 5.1.4.

Page 99 of 165

Figure 5-5: Digester influent and effluent mass fluxes

Table 5-4 shows that the reduction in biodegradable organics in THP digestion of 75% was greater than that in conventional digestion of 68%. This was most likely due to the increased rate of digestion, as discussed in Section 4.5.2. The unbiodegradable organics remained unchanged in the case of conventional digestion. However, as discussed in Section 4.3.4 THP increases biodegradability by converting some of the unbiodegradable organics (UO) into biodegradable organic (BO) substances, as well as converting some BO into unbiodegradable soluble organics (USO). Therefore, in THP digestion the change in biodegradable organics of 75% reduction in fact was greater over AD step, as some of the UO was first converted to BO in the THP before entering the digester where the reduction in BO occurred. The inorganic ISS was reduced due to the release of PP (which registers as ISS in solids tests), and even though ISS was generated due to the precipitation of struvite, the overall net impact on ISS was a 4.2% decrease in conventional digestion and 3.4% decrease in THP digestion. The ISS drop in THP digestion was less due to the fact that more struvite was formed per mass of PP released, as discussed in Section 5.1.3.

Table 5-4: Solids breakdown of digester influent and effluent

Page 100 of 165

Conventional digestion

influent

Conventional digestion

effluent

% change

THP digester

influent

THP digester

effluent

% change

kgTSS/d kgTSS/d kgTSS/d kgTSS/d

Biodegradable

organics 26152 8457 -68% 66545 16574 -75%

Unbiodegradable

organics 20108 20108 0% 51166 45909 -10%

Inorganics 14204 13607 -4.2% 36144 34924 -3.4%

Total 60463 42171 -30% 153855 97407 -37%

The guidelines as set by WRC (Herselman and Moodley, 2009) discussed in Section 2.2 state that the treatment of sludge requires a volatile solids reduction of at least 38% on a 90- percentile basis for treated sludge to be classed as a Class 1 stability sludge. This is the highest stability class and allows for more disposal options, as discussed in Section 2.3.

Volatile solids reduction is calculated using O’Shaunessy’s formula can be used to calculate the volatile solids (VS) reduction in a digester. The below calculation is taken from the WRC guidelines Appendix 2.

𝑉𝑆 𝑟𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛 (%) = (𝑉_𝑖− 𝑉_𝑜

𝑉_𝑖 − 𝑉_𝑖⋅ 𝑉_𝑜) ⋅ 100 (5-1)

Where,

• Vi = volatile fraction in feed sludge, dimensionless

• Vo = volatile fraction in digested sludge, dimensionless

• The volatile fraction is given by the ratio for each stream mgVSS.l^-1/mgTSS.l^-1

As can be seen in Table 5-5 the volatile solids reduction of both conventional and THP digestion are 36% and 45% respectively. THP digestion comfortably exceeded that required of the WRC guidelines for Class 1 of 38%. However, for conventional digestion the VSR does not comply with Class 1. Further, the VSR for conventional digestion is low compared to that found in Section 2.4.3 of the literature review for mixed sludges of 45%. The VSR in THP digestion is also lower compared to that typically found in literature (see in Section 2.7.3) of 48% to 54%. The lower VSR experienced in this study are most likely due to the influence of ISS in changing the VSS/TSS ratio. In digestion of ND WAS and primary sludge ISS stays relatively unchanged and only VSS is altered by the AD. The digestion of NDBEPR sludge releases PP which reduces ISS, while at the same time precipitating struvite, which again increases ISS. As discussed in Section 5.1.3 the reduction is ISS due to PP release is greater than the subsequent increase due to struvite precipitation. Thus, the net reduction in ISS creates a higher VSS/TSS ratio in the final digested sludge than if ISS had remained unchanged.

The biodegradable fraction of the sludge is the biodegradable organics (XB) divided by the total solids (XT). The biodegradable fraction of the digester effluent has been reduced from 43% to 20% in the case of conventional digestion and 43% to 17% in the case of THP digestion. Note in THP digestion the sludge biodegradability was first increased due to the THP process (discussed in Section 4.3.4) before the AD step reduced the biodegradable organics. Therefore, as discussed previously the reduction in AD biodegradable organics was

Page 101 of 165

possibly even better than that stated here. This can be supported by the improved overall COD reduction of THP digestion of 47% compared to that of conventional digestion of 38%.

THP also showed a better overall solids reduction of 37%, mostly due to a reduction in organics shown in Table 5-4, where conventional digestion achieves a 30%. However, for the purposes of sludge stabilisation the final sludge mass’s biodegradable fraction is the focus and THP digestion still provided a lower final sludge biodegradability. The lower the biodegradable fraction then the more suited the sludge is for application to land, and thus the final sludge from THP digestion is of a more stable quality than that from conventional digestion.

Table 5-5: Solids treatment and stabilisation

Conventional THP

Solids flux

Influent solids flux 60463 153855 kgTSS/d

Effluent solids flux 42171 97407 kgTSS/d

Influent VSS/TSS (Vi) 0.77 0.77

Effluent VSS/TSS (Vo) 0.68 0.64

Volatile solids reduction (O’Shaunessy’s formula) 36% 45%

Total solids reduction 30% 37%

Biodegradability

Influent biodegradable fraction to digester 43% 43% XBi/XTe Effluent biodegradable fraction from digester 20% 17% XBe/XTe

COD

Influent COD load 68 174 tonCOD/d

COD reduction 38% 47%