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Sludge Management

Safe treatment of wastewater is one of society’s most fundamental challenges. Wastewater treatment creates sewage sludge, the processing of which is highly energy-intensive, contributes to greenhouse gas emissions, and is still far from efficient. The impact of sludge management systems has to be carefully addressed in the context of EU energy, climate change mitigation, and resource efficiency policies.

The major challenges include:

  1. Source control of pollutants
  2. Material reuse or disposal
  3. Environmental and health impacts of sludge management practice
  4. Regulatory compliance and public perception

The END-O-SLUDG project addressed these difficult challenges with a holistic approach. Rather than tackling each issue individually, the END-O-SLUDG project considered sludge production, treatment, product development, recycling and the environment as an integrated system. In doing so the END-O-SLUDG project has provided a toolkit of novel processes and methodologies that can be used to design the most efficient sludge management solution suited to the local situation.

The following articles provide an overview of some of the novel processes and methodologies that the END-O-SLUDG team covered:

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Pre-treatments: Ultrasonication and Enzymic Hydrolysis

END-O-SLUDG partners Uniovi provided research into the effects of Ultrasonication on sludge hydrolysis. Ultrasonication was applied to different sludge samples (primary, secondary, and mixed) in order to study the solubilisation of the substrates. Soluble Chemical Oxygen Demand (sCOD) and ammonium nitrogen were employed to test their response to ultrasound. Chemical characterization was performed at an initial stage (raw sludge), just after the pre-treatment, and then again after 24 hours of further fermentation at 37 degree Celsius.

Figure 1: Ultrasonication equipment

Figure 1: Ultrasonication equipment

Five energy levels were applied to the samples and monitored: 3,500, 7,000, 10,500, 14,000 and 21,000 kJ/kgTS. The main findings may be summarised as follows:

  • As a general rule, the greater the energy input, the higher the hydrolysis effect.
  • Reductions in soluble COD and ammonium nitrogen were sometimes observed after fermentation. These might be explained by the removal of organic compounds and ammonia.
  • Temperature effects had much more influence on increases in NH4+-N than ultrasound.
  • Much higher solubilisation of organic matter was obtained when applying ultrasound to secondary sludge.
  • Energy inputs of 7,000 kJ/kgTS achieved increases in soluble COD of up to 342 fold.
  • The higher solubilisation rate does not always mean higher biodegradation.

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Pretreatments: Inverted phase fermentation

END-O-SLUDG researchers have tested new processes that have the ability to increase biogas yield during the secondary treatment processes. During sludge treatment the energy contained within the sludge is retrieved, usually in the form of biogas that can then be used for heating and power. Whilst traditional treatment methods only release 45% of the potential biogas available, END-O-SLUDG partners have researched improved treatments that have the potential to yield even higher levels of biogas. By using inverted phase fermentation, watery sludge could be split into separate solid and liquor fractions (Fig. 2 below) that could then be converted to biogas in smaller more efficient digesters therefore reducing the capital investment considerably.

Figure 2: Inverted phase fermentation

Figure 2: Inverted phase fermentation

For instance, the retention time of the liquor in the digester was reduced to 3-1 days achieving biogas yields rates up to 3.3 m3/m3digester┬Ěday. Furthermore, it has been demonstrated that the digested solid fraction could be treated by fine grinding to break down the sludge matrix further releasing biogas during a secondary digestion resulting in 75% overall organic to energy conversion.

elena_video

Please click here to view a short video by Prof Elena Maranon on work in this area

Further reading: The following reports of the research undertaken in this area have now been published:

L. Negral, et al., Short-term evolution of soluble COD and ammonium in pre-treated sewage sludge by ultrasound and inverted phase fermentation, Chem. Eng. Process. (2013), http://dx.doi.org/10.1016/j.cep.2013.02.004

E. Maranon, et al., Influence of conditioning agents and enzymic hydrolysis on the biochemical methane potential of sewage sludge, Water Science and Technology. (2013), http://www.iwaponline.com/wst/06807/wst068071622.htm

L. Negral, et al., Inverted phase fermentation and ultrasound pre-treatments of sewage sludge: Biochemical methane potentials, Journals of Residuals Science & Technology. (2013), Still in preparation

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Micro-milling

Scientists at United Utilities used a micro-milling technique to test the viability of producing marketable derivatives from sludge. Digested sludge residue represents about 55% of the total organic content within the biomass and micro milling is a process that can be used to access the valuable products contained within it.

Understanding the characteristics of the different components of the biomass was vital if full industrial exploitation of this resource was to be made.

Figure 3: The main components in a bacterial cell structure

Figure 3: The main components in a bacterial cell structure

The bacterial cell wall (figure 3 above) of the sludge biomass is not readily biodegradable as it provides a protective shell for the intracellular content, and an ability to break down the cell wall to reach the protected materials is essential for this challenge.

Figure 4: Micro milling preparation of sludge Zr02 beads

Figure 4: Micro milling preparation of sludge Zr02 beads

Scientists at END-O-SLUDG showed that by using the micro milling process (figure 4 above), it was possible to breakdown the bacterial cell wall to recover a biopolymer product known as Biopol. Biopol is a polyelectrolyte with many interesting properties and potential applications. By evaluating its use for the co-precipitation of phosphate from wastewater the scientists found it to be an excellent alternative to struvite formation as a method for phosphorus recovery from sludge liquor. Owing to the fact that Biopol itself has significant phosphorus content (figure 5 below), the co-precipitated product has an enhanced P content (24.3%), which compares favourably with struvite (26.9%).

Figure 5: The chemical structure of the calcium-phosphate DNA co-precipitate

Figure 5: The chemical structure of the calcium-phosphate DNA co-precipitate

The co-precipitate had another key advantage as a fertiliser due to the labile nature of Biopol, which would lead to rapid mineralisation of the nutrients making them readily available to growing crops. The production of Biopol as a new form of sludge derivative has the potential to provide a highly effective mean for sludge management; enabling the targeting of a phosphorus-rich product at farms most deficient in the nutrient, and nutrient-deficient sludge to arable land where organic matter tends to be minimal.

The work undertaken at United Utilities represents an important step in the future of sustainable sludge management. Previously an untapped resource that is either incinerated or spread to land, sludge residue could provide an abundant source of raw material suitable for the production of speciality chemicals and energy.More information on Biopol research undertaken by United Utilities can be found on our technology page.

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Competitive Exclusion

E. coli regrowth in digested sludge cake is an issue that affects the entire wastewater industry. Although current sewage sludge treatment processes are certainly capable of significantly reducing the numbers of micro-organisms to acceptable levels, it has been found that the indicator micro-organism E. coli is still capable of regrowth after the sludge has been treated and dewatered.

Effective solutions to this phenomenon are urgently required in order to maintain stakeholder confidence in the quality of the final product that is destined for agricultural recycling.

A team of researchers at Cranfield University and United Utilities investigated new ways of tackling E. coli regrowth through the application of a novel treatment that uses pro-biotic cultures to combat the phenomenon.

The first step for the team was to recreate the E. coli regrowth phenomenon under controlled laboratory conditions. Once this was successfully achieved the team moved on to identifying suitable pro-biotic cultures to test. These were ‘friendly’ bacteria that are capable of growing in sludge products and have the ability to slow down the growth of E. coli by either competing for the food supply or by producing antibiotics. The friendly bacteria are natural and harmless to humans and will themselves eventually die-off when the sludge nutrients are used up. This approach is similar to the idea of consuming so-called ‘probiotic’ or ‘live’ cultures in fermented dairy products like yoghurt to promote a healthy digestive system.

Figure 7: Probiotic experiments

Figure 7: Probiotic experiments

Experiments showed that a competition effect could be observed in the lab. In the image above (figure 7 above), the plate on the far left is a culture of E. coli with no competition. The plates in the centre and the right have competitor bacteria added to the E. coli. There are fewer green dots (colonies) on the plates with the friendly bacteria added indicating that the competitors had affected the E. coli growth. Work is continuing to find the most effective way of maximising the competitive effect before trials on a large-scale system.

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