Reducing the environmental impact of wastewater screenings and producing valuable by-products through the application of anaerobic technologies

Wastewater screenings are a waste material that, without any possibility of recycling or reuse, currently are mainly landfilled or incinerated. Nevertheless, such material containing high percentage of organic matter, and thus a potential generator of GHGs, should ideally be diverted from landfill. Therefore, more environmentally-friendly disposal methods of screenings are urgently needed. This situation, coupled with the ongoing responsibility of the wastewater sector to reduce its carbon footprint and increase the use of renewable energy, prompted the idea of applying anaerobic digestion to screenings. Thus, by applying batch tests it was possible to establish the feasibility of digestion of screenings with a methane potential of 0.335 m3/kg VSapplied. Methane production over time could adequately be described by first-order kinetics and the hydrolysis rate constant was in the range of 0.061-0.173 o'. The differences in biodegradation rates from the materials contained in screenings were evidenced by a diauxic growth which could be simulated by the modified Gompertz non-linear model. A conventional CSTR process with an OLR of 2.5 kg VS/ m3/day and an HRT of 15 days produced a methane yield of 0.405 m3/kg VS applied , and showed that screenings have the sufficient buffer capacity to be digested without the need for pH correction. However, operational problems such as foaming and accumulation of solids severely limit the process from being taken to full-scale. Therefore, a flexible two-stage process was proposed, including a batch first stage with a leach bed reactor (LBR), which allows the acid digestion of screenings without any pretreatment or pH adjustment. Furthermore, a recirculation ratio of the leachate of 8 L/L reactor/day increased the acid I production by 11 % compared with 4 L/L reactor/day, and recovered 60% of nitrogen and 70% of phosphorus. It was also found that due to the content of biodegradable and cellulolytic materials in screenings their acidogenesis is inhibited at pH below 5.5. The co-digestion with FW favoured the hydrolysis of screenings while the co-digestion with the OFMSW increased the nitrogen recovery by 23%. When the LBR was coupled with a methaniser in a two-stage process, the methaniser was very stable, removing on average 90% of COD and 85% of VFAs and producing a maximum biogas yield of 0.410 m3/kg COD. This two-stage process allowed the recovery of 95% of the energy embedded in screenings, with 45% being transferred to the biogas. Thus, the digestion of screenings within an existing WWTP presents a positive energy balance; given that for each tonne of screenings digested there would be a gain of 490 kWh. This energy gain means that the digestion of screenings through the technology proposed herein would be not only an ecologically safe management alternative (saving 50 kg C02e per tonne of screenings digested), but also could become a cost-effective technology within the wastewater industry.EThOS - Electronic Theses Online ServiceGBUnited Kingdo