Influence of wastewater treatment on sludge production and processing

Traditionally, wastewater treatment works (WWTPs) have been designed to meet legislative drivers based on removal of a number of contaminants prior to discharge back to the natural environment. Treatment of sewage sludge, an inevitable by-product of wastewater treatment, is complex and can account for nearly half of the total costs of wastewater treatment. Sludge contains an inherent amount of energy which can be used directly via mono- or co-firing with other materials (and/or fuels) or, indirectly by producing biogas, a renewable energy, which can be converted into electricity and heat using co- generation equipment. The biogas can be further processed for introduction onto gas grid networks or use as vehicle fuel. In addition to energy, biosolids contain a quantity of valuable materials that can be exploited. Thus, they are often used as a fertilizer where they can displace fossil-fuel-based (synthetic) fertilizers. However, unlike the latter, biosolids contain a number of other beneficial soil improving attributes such as: carbon; organic material; moisture, and structure to aid with drainage. Their use can substantially reduce carbon footprint. A recent study has shown that offsets in excess of 1 tonne CO2e/t dry biosolids can be achieved used, dependent on application (ANZBP 2012).

However, in spite of the large costs attributed to processing sludge, little attention is afforded to the impact that wastewater treatment has on the type and quantity of sludge (and subsequent biosolids) produced.

Typically, wastewater, industrial effluent and rainwater run-off enter a sewage works where they are initially screened prior to going onto primary treatment. During this stage solids and other materials settle out and form primary sludge. The supernatant (wastewater) from this stage passes to a second stage of treatment where nutrients are removed. This is achieved through facultative and aerobic bacteriological reactions in the presence of air/oxygen, and accomplished in a large variety of configurations. The solid material leaving secondary treatment constitutes mainly bacteriological material and is known by a number of names including: secondary; bacteriological; waste or surplus activated sludge (WAS or SAS).

The production of secondary sludge is fundamentally influenced by the quantity of time required for aeration – known as the sludge age. As wastewater standards tighten to remove further nutrients, longer sludge ages are required to accommodate the less favorable kinetics of the nutrient removing bacteria (Painter 1986). The impact of sludge age on sludge yield was determined using McCarty's methodology (1966) and the results are shown in Figure 1.

Figure 1 shows sludge production decreasing with extended sludge age. Sludge yield drops from 0.78 kg/kg via 0.66 kg/kg to 0.51 kg/kg under carbonaceous, nitrifying, and extended aeration conditions. Therefore, as legislation tightens, secondary sludge production decreases.

If phosphorus removal is required in addition, the wastewater must undergo further processing. Removal is based on normal uptake into the sludge which is then wasted (resulting in a reduction of 20–30% phosphorus); use of chemicals to form precipitates, or by enhanced biological uptake.

While chemical precipitates of phosphorus salts add sludge volume they are largely inert and may consume capacity of existing downstream infrastructure. The addition of chemicals also reduces the volatile fraction of the sludge, which reduces its energy content. A study to determine the suitability of digested sludges for incineration (Barber 2007), showed that ferric chloride addition increased ash content by 10% compared to un-conditioned digested sludge. Elemental analysis revealed an unwanted consequence caused by chemical addition by which oxygen content increased by 15% (probably due to precipitation of hydroxide salts) and reduced calorific value further.

With enhanced biological uptake, conditions in the WWTP are manipulated to increase bacteriological uptake approximately 2–3 times metabolic requirements (Rittmann & McCarty 2001). However, this process has been known to adversely influence downstream sludge processing, such as reduced biogas production from anaerobic digestion (approximately 30% – Rybicki 2003), and worse dewaterability (Barnard & Shimp 2013).

Figure 2, collated from various sources (Vesilind 2003; van Haandel & van der Lubbe 2007; Speece 2008; and Asaadi 2008) and sample analysis, shows the typical composition of the organic fraction of primary and secondary sludge.

The fundamental differences between primary and secondary sludge are evident in Figure 2. Compared to secondary sludge, primary sludge has more lipids and fiber but less protein and phosphorus. The molecular formulae for these sludges are roughly:

• Primary sludge C₂₃H₃₅O₈N (C:H:O:N = 61:8:28:3)

• Secondary sludge C₇H₁₁O₃N (C:H:O:N = 53:7:31:9)

These molecular formulae suggest calorific values of approximately 25,700 for primary and 21,800 kJ/kg for secondary sludge as volatile substance. The greater carbon content (primarily from the lipids) results in primary sludge containing approximately 15% more energy than sludge formed during aeration.

It is widely recognized that these sludges react differently to downstream processing. The improved digestibility of primary sludge over secondary was reported as early as 1926 (Zack & Edwards cited by Gossett & Belser 1982). Numerous texts summarize the difference in volatile solids' degradation during anaerobic digestion, with proportions in the range 55–60% and 30–45% degraded for primary and secondary sludge, respectively (WEF 1987).

Using the molecular formulae above, the theoretical biogas yield per kg volatile matter destroyed can be determined. Calculations show 0.983 and 0.788 Nm3 biogas/kg VS destroyed for primary and secondary sludges. These figures compare well with literature values of 900–1,000 for primary and 700–800 l/kg VS for secondary sludge (Kopp 2003; Winter & Pearce 2010). In normalized terms, biogas production per kg secondary sludge destroyed is only 80% of that for an equivalent amount of primary sludge.

As noted, tighter wastewater treatment regulations result in increasing sludge ages, which affects the digestibility of the sludges. Gossett & Belser (1982) studied the impacts of sludge age on secondary sludge digestibility at 15 days HRT. They found that volatile solids destruction fell from 30 to 25% between 5 and 10 days sludge age and to less than 15% at 30 days.

As for digestion, it is well known that primary sludge is far easier to thicken and dewater than that from aeration processes (ASCE 2000; Kopp 2003). This has been attributed to numerous reasons, one of which is the gelatinous nature of secondary sludge (ASCE 2000) as this influences the quantity of bound water which cannot be removed by standard dewatering. To measure bound water content in sludge, Heukelekian and Weisberg (1956 – cited in ASCE 2000) determined a bound water level of 3 g/g dry solids for sludge. In later work, Katsiris & Kouzeli-Katsiri (1987) found similar results for digested sludge at 2–2.5% dry solids, but found levels of 9–12 g/g in biological sludge at 0.4%.

In summary, tighter legislation leads to a need for additional aeration and chemicals, making sludge harder to process downstream. This worsens with increasing sludge age. To quantify the impacts of tighter legislation a model was developed to determine sludge production and impact on downstream unit operations.

Initial data on sludge yield, oxygen requirements, and ammonia release or consumption were determined using methods described by McCarty (1966), and recalculated for different conditions and sludge ages. The energy for aeration was determined by modeling various secondary treatment configurations based on activated sludge, using standard equations and methods, and equipment suppliers' information.

Phosphorus removal by natural wastage was calculated using Rittmann and McCarty's equation (2001). More phosphorus was removed, if necessary, by adding metal salts based on typical requirements. Sludge production was determined stoichiometrically.

Calorific values were determined using Dulong's equation (Technical Report, CEN/TR 13767, 2004) and chemical composition data from the literature.

Downstream digestion performance was calculated using a model which combines correlations obtained from full-scale plant data and kinetics. This predicts performance based on: sludge type and age; digester operating temperature and retention time, and quantity of dead space (Barber 2005).

Downstream dewatering performance was based on a mixture of operational data from numerous sludges and analysis of literature results.

The model's baseline conditions are given in Table 1:

Several options were modeled to determine the impact of tighter legislation.

Figure 3 shows the impact of wastewater configuration on sludge production, while Figure 4 shows both the energy generated from co-generation by burning biogas generated from anaerobic digestion, and that consumed during wastewater aeration. It also shows the net overall energy balance, i.e. the difference between energy generated and consumed.

This may be observed by comparing baseline option 1 to option 2 (enhanced primary treatment) and option 3 where primary treatment is bypassed to provide carbon for nutrient removal. In terms of sludge produced, it is evident that increasing the capacity of primary treatment increases sludge production. Enhancing primary treatment results in an increase of 15% over the baseline, although the combined sludge contains 30% less secondary sludge. When primary treatment is omitted, sludge production is 45% less than in the baseline. Although option 2 produces more sludge, the aeration demands for secondary treatment are lower than the control, due to the redirection of load away from aeration and into anaerobic digestion. Enhancing primary treatment from the baseline results in a better energy balance, because of a combination of almost 25% more biogas energy (due to a greater proportion of primary sludge) and aeration energy reduced by 22% (removal of additional load prior to aeration). In fact, in only one other option studied is the energy generation potential from digestion greater than the demand for aeration. That option is based on a relaxed consent (option 4). This highlights the importance of enhancing primary treatment if site-wide energy demands are to be reduced.

However, the opposite is true when considering nutrient removal seen in option 3. Here, energy demand increases by between 2 and 3 times. The increase in overall demand is due to increased aeration needs and a drop in anaerobic digestion performance, resulting in 60% lower energy generation. This is due mainly to the absence of primary sludge, but also due to the poor digestibility of the secondary sludge paired with lower biogas yield on a unit basis, and nutrient limited conditions. When Alphenaar et al. starved UASB reactors of phosphorus, methane production fell by over 50% compared to a control with adequate phosphorus available (1993). While the drop in performance was bacteriostatic, phosphorus levels had to be increased to over 5 mg/l before methanogenesis returned to previous levels. Diversion of carbon from digestion further reduces biogas production as the digester is fed a larger proportion of less-readily biodegradable COD. Winter & Pearce (2010) investigated the use of sludge storage to produce volatile fatty acids for nutrient removal prior to anaerobic digestion. Under ambient conditions relative to the UK, they found maximum BOD release of up to 50 mg VFA/g DS activated sludge stored, which peaked after approximately 2 weeks. Although no breakdown of the volatile fatty acids was given, this release could account for approximately 5% additional biogas loss based on the assumption that it comprised equal quantities of acetate, propionate and butyrate.

This can be observed by comparing the baseline option to options 4 (no nitrogen consent), 5 (tighter consent), and 6 (no phosphorus consent). Options 4–6 contain the same quantity of primary sludge, however the quantities of secondary and chemical sludge change as a result of differences in sludge age. As seen in Figure 1, yield decreases as sludge age increases, explaining a 10% increase in option 4 over the baseline. In option 5, secondary sludge production decreases slightly due to tighter ammonia standards, but the main difference arises due to tighter phosphorus standards. Reducing effluent levels from 2 to 1 mg/l increases chemical sludge by 20%. The volatile solid content rises by 10% compared to option 1, with a similar increase in calorific value, as there is no additional inert material in the sludge.

Tightening consent also fundamentally influences the overall energy balance. If regulation requires only carbon removal (option 4) the overall energy balance is positive. This is due to a reduction in aeration exceeding 45% and a concomitant reduction in activated sludge. The latter increases biogas yield by 5–10%, with energy recovery of nearly 200% compared to the baseline. As standards tighten, aeration needs increase resulting in additional activated sludge as demonstrated in option 5. The model predicts an increase in biogas production of 5% due to the additional sludge, but at the expense of a 10% increase in aeration requirements. These factors combined result in a 40% increase in energy requirements compared to baseline.

By contrast, if consent is relaxed, as in option 6 (no phosphorus removal), energy requirements are 25% lower. As aeration requirements remain uniform, this arises from increased biogas production due to the absence of additional inerts consuming digester capacity, increasing SRT and thus digestion capacity.

Cake production was determined using a combination of digester performance (as ‘total solids destruction’) and the predicted dewaterability of the digested biosolids. The quantity of cake affects downstream processing and transport – see Figure 5.

The lowest cake production corresponds to option 3 which has no primary treatment. However, due to the lack of primary sludge, solids destruction levels and dewaterability are also the worst and this reduces the benefit of lower sludge production. While absence of primary treatment reduces raw sludge production on a dry basis by 45%, cake production is only 20% less. Other options showed little difference compared to baseline.

Differences in dewaterability lead to water content variation that fundamentally affects any downstream thermal processes, such as drying prior to land application or incineration Although cake production was 20% less than baseline, when primary treatment was absent, water evaporation requirements (if required to dry to 90% solids) were only slightly reduced, and would be similar as sludge age extended. The baseline option requires approximately 2.5 t water evaporation/t raw dry sludge produced, but this increases to 4 t/t if primary treatment is omitted. Enhancing primary treatment improves digester performance, which reduces downstream water evaporation requirements to approximately 2 t/t.

The reduction in sludge cake in the absence of primary treatment (option 3) coupled with high water content, makes these sludges poor candidates for downstream energy recovery, with approximately half the biosolids energy compared to the baseline in absolute terms. This was noted in a previous work (Barber 2014) which showed lower energy recovery from secondary sludge during both anaerobic digestion and incineration. The reduction in digestion was due to poorer digestability and this resulted in a higher energy content cake going to incineration. However, as the dewaterability is also poor, a large diversion of energy away from power generation at the incinerator is required to dry the sludge to eliminate auxiliary fuel requirements. In contrast, enhancing primary treatment, which increases sludge production, and employing a carbonaceous configuration (option 4), increase energy content on an absolute basis by 10–15% compared to the baseline.

1. Tightening wastewater standards increases the proportion of secondary and chemical sludge relative to primary sludge. This will make sludge processing increasingly difficult in future.

2. Primary sludge contains more lipid and fiber than secondary sludge, while the latter has higher concentrations of protein, nitrogen and phosphorus. These differences result in primary sludge having inherently higher energy content, 20% greater biogas yield per kg destroyed, and fewer nutrients to remove.

3. Secondary sludge is less amenable to anaerobic digestion and dewatering than primary sludge, and becomes less responsive as sludge age increases further.

4. Removal of primary treatment for nutrient removal has a number of negative downstream impacts. These include increased aeration requirements of >20%, and a reduction in biogas production of approximately 60%, due to a combination of consumption of readily biodegradable carbon during nutrient removal, loss of primary sludge and nutrient starved conditions in the digestion stage.

5. A positive energy balance – i.e., energy self-sufficiency – is potentially possible for systems designed for carbon removal or systems employing enhanced primary treatment.

6. Enhancing primary treatment reduces downstream aeration requirements by more than 20% and increases biogas production by 25% compared to standard primary treatment.