Practical use of new system for highly efficient recovery of energy from sewage and garbage

Conventional sewage treatment plants in Japan have relied on activated sludge processes, in which a great deal of aeration power is used along with dewatering to reduce sludge volume. In Japan in 2008 about 7.2 billion kWh of electric power were used in sewerage operations. That was about 0.7% of the country's total power consumption (Japan Sewage Works Association 2011). It is increasing annually as the sewered percentage of the population increases. The energy supply situation in Japan has grown tighter due to the impact of the nuclear accident in Fukushima, and creating energy from biomass, a future alternative natural energy, has become an urgent task.

A new system, in which influent is subjected to through solid–liquid separation, followed by aggressive approaches to saving energy in water treatment and creating energy for sludge. The system positively incorporates the use of garbage as an energy source, a possibility that has been ignored in conventional sewage treatment. Sewage treatment plants can enhance the convenience of urban life by accepting garbage and ground waste from disposers and utilizing them positively as energy sources to increase generation of biogas as a renewable energy source, thereby constructing plants with higher energy self-sufficiency ratios.

Against this background, a collaborative research team made up of personnel from METAWATER and the Japan Sewage Works Agency has been engaged in ‘Demonstration study of an energy management system using intensive solid–liquid separation technology’ at the sewage treatment plant in Osaka, Japan. This work was commissioned by the National Institute for Land and Infrastructure Management under its sewerage innovative technology demonstration (B-DASH: Breakthrough by Dynamic Approach in Sewage High Technology) project. The system technology is designed to perform energy management for the whole of a sewage treatment plant. The system uses intensive solid–liquid separation for maximizing raw sludge recovery, thermophilic digestion for maximizing methane production, and smart power generation system for maximizing the power generation efficiency and minimizing power consumption. Each of the technologies can reduce Greenhouse Gas (GHG) emissions and costs. The three technologies are combined into a system to enhance the GHG emission and cost reduction. The features of technologies are as follows:

(1) Intensive solid–liquid separation

Intensive solid–liquid separation saves energy consumption by fully collecting raw sludge with high gas generation rate capacity from sewage to reduce the load on water treatment.

(2) Thermophilic digestion

‘Thermophilic digestion’ refers to the highly efficient recovery of biogas from raw sludge and garbage.

(3) Smart power generation system

The smart power generation system consists of plant operation optimization control and a hybrid fuel cell to promote energy conservation, energy creation, and energy storage for an entire sewage treatment plant system (see Figure 1 and Table 1).

Several researchers observed the best performance in terms of decomposition rate and digestion gas generation with substrate having Total Solids (TS) amount ratio in the range of 1:4 (Diaz et al. 1981; Demirekler & Anderson 1998), or that of 1:0.3, respectively (Hamzawi et al. 1998)

The demonstration facility was set up in the Osaka City's Nakahama sewage treatment plant. It used 5,700 m³ of sewage per day, which was part of the treatment plant's influent sewage. The garbage used included uncooked refuse from hotels and department stores in the city and cooked garbage (leftover food) from hospitals. It was brought to the demonstration facility by garbage transporters for segregation and subsequent loading into the digester. Table 2 shows the composition of the raw sludge and garbage. Raw sludge was added to the garbage to obtain mixtures with set sludge/garbage ratios such as 1:0.7, 1:0.5, or 1:0.2. The loading volumes were maintained at a constant rate of 12 m³/d. All of compositions were determined in accordance with standard method (Japan Sewage Works Association 2012).

The aim of this technology is to achieve energy conservation by reducing the load on water treatment by thoroughly recovering raw sludge which can generate large amount of digestion gas from sewage. The facility is designed to separate wastewater into filtrate and backwash reject water (raw sludge). It has multiple tanks for primary storage and thickening of raw sludge. The separation process is divided into a filtration process and a backwash process. In the filtration process, wastewater is put into an upward flow through a filter media bed located below the upper screen. Suspended solids (SS) and other pollutants are removed to generate the filtrate. In the backwash process, which is designed to recover raw sludge, the high-velocity downward flow washes out SS trapped in the filter media. As shown in Figure 2, the filtrate captured in the upper screen is flushed down by a high-speed downward water flow. This is done by automatically and opening the valve provided in the lower portion of the tank for rapid, short-term backwash. In the primary thickening tank, thin raw sludge (TS 0.1%) is thickened to 1% through gravity settling. Raw sludge is further thickened to about TS 3% in the secondary thickening tank.

This is carrier-filled-type wet thermophilic digestion, which is used to substantially downsize the digester volume of the sewage treatment plant from the conventional level. The digester is packed with a non-woven immobilization carrier, to which anaerobic micro-organisms are allowed to adhere to increase the micro-organism concentration in the digester and to withstand load fluctuation. Compared with ordinary mesophilic digestion (HRT 20 days), thermophilic digestion can reduce HRT by 1/2–1/4 or less (WEF & ASCE 1992; Ghosh et al. 1999; Zabranska et al. 2000). This in turn enables downsizing of the digester volume and construction of the digester from steels, rather than from reinforced concrete, which is conventional material (Hobson & Wheatley 1993). The technical advances described above results in reduced construction cost.

The smart power generation system consists of a plant operation optimization control system and a hybrid fuel cell. The former supports energy conservation, production, and storage for the sewage treatment plant as a whole. The system is adjusted to enable leveling of the demand on the basis of previously set functions. For power supply through generation, overall commercial input supply/demand is leveled by a function that predicts digestion gas generation. The gas is stored with digestion gas storage equipment. The hybrid fuel cell includes a reformer and a phosphoric acid fuel cell stack. It features generation efficiency of 40% or more, which is the highest efficiency of any gas power generation system (Kasahara et al. 2000). Within a range of 30–100 kW, high-generation efficiency can also be achieved with a partial load supply, and combined use of the digestion gas and city gas is possible. If the digestion gas generated in the sewage treatment plant is not enough to meet the amount required, the hybrid function enables replenishment by city gas to make up for the shortage. Theoretically, the full amount of digestion gas can be utilized.

This process is designed to efficiently recover raw sludge in influent sewage. It substantially contributes to increasing the overall removal rate of SS in the influent – a rate much greater than that achieved by a primary settling tank. The relationship between the influent raw sewage SS concentration and the removal rate is shown in Figure 3. Evaluation in terms of the given filtration rate shows a trend that, for SS, the removal rate rose with increased raw water concentration. Also, when the filtration rate was changed, the removal rate tended to increase with a decreasing filtration rate. Note that, with a filtration rate of 250 m/day, the average SS removal rate was 66.5% (raw water SS concentration: 37–280 mg/L, average: 117 mg/L). Accordingly, the relationship between the soluble BOD concentration in raw water and the soluble BOD concentration in water from solid–liquid separation was studied. As illustrated in Figure 4, most of the soluble BOD in raw water migrated toward the water from solid–liquid separation without being treated.

Loading of raw sludge was started on 10 July 2012. It reached the rated HRT 5 days on 25 July 2015. Loading of garbage was started on 1 August; it reached the rated HRT 5 days at the specified TS amount ratio (1:0.7) on 9 August. Subsequently, the TS amount ratio (garbage to raw sludge) was changed gradually. The result is shown in Figure 5. Under the experimental conditions (a total of 61 days) at the rated load (raw sludge: garbage = 1:0.7 (VS load of 6.0 kg/m³/day), HRT 5 days), the VS decomposition rate was 70.0% on average while the digestion gas generation was 759 Nm³/t-loaded VS on average. Subsequently, even when the TS amount ratio was changed, stable operation continued without bubbling or accumulation of organic acids and ammonium nitrogen, etc. The experiment was performed using raw sludge only from April 2013. The VS decomposition rate, digestion gas generation and methane concentration were 62.6%, 550 kg/m³/day and 55% respectively. Average digestion performance was calculated for cases of raw sludge only, 1:0.2, 1:0.5, and 1:0.7. Figure 6 shows the decomposition rate and digestion gas generation relative to the garbage mixing ratio (TS rate) in the total substrate. The approximation in Figure 6 shows that the data can be utilized in the future planning and design.

The gross generation with fuel cells during the demonstration period (21 February 2012–7 March 2013) was 216,948 kWh, and the average generation efficiency was 40% (at the transmission end) when calculated from the total digestion gas consumption of 50,136 Nm³ (with average methane gas concentration at 59.9%) and from the total city gas consumption of 16,296 Nm³. The result at constant fuel cell output was obtained by setting the generator output at a constant 30 kW and the target value during demand control at 50 kW. The effect of reducing purchased power was determined to be at a reduction rate of 55% because power consumption in normal conditions was 1,648 kWh (converted to daily consumption), the demand control was 39 kWh, and from power generation of 871 kWh.

On the basis of the data obtained with this demonstration test equipment under the operation of 5,700 m³/day, an estimate was made for an assumed sewage treatment plant of a scale of 50,000 m³/day (with an influent sewage SS concentration of 145 mg/L, and BOD concentration at 170 mg/L). The garbage mixing rate was 0.6 to a raw sludge TS amount 1. It was assumed that, after introducing the innovative system, the sewage treatment plant would feed highly digestive substrates such as raw sludge and garbage to thermophilic digestion, to replace the primary sedimentation tank, and that intensive solid–liquid separation would be used for primary sedimentation and for fuel cell power generation, as shown in Figure 7.

For the estimate, it was assumed that the garbage conventionally incinerated would be fed into the sewage sludge treatment as biomass. In this way, the incineration plant would be used as a waste treatment plant without garbage, and construction and treatment costs would be reduced.

The introduction of intensive solid–liquid separation (shown in Figure 7) causes an increase in raw sludge and a decrease in excess sludge difficult to decompose in digestion. Moreover, the feeding of garbage enabled recovery of a large amount of digested gas in a compact digester in five digestion days.

Applying thermophilic digestion enabled reducing digester volume for digestion of raw sludge and garbage to 1/4 or less that needed in mesophilic digestion.

The SS removal rate achieved with the demonstration equipment was higher in intensive solid–liquid separation than in the primary settlement, and the raw sludge recovery was larger substantially in the equipment after introduction the technology (4.4 ton-DS/day) than conventional one. Also, the excess sludge recovery was lower in the equipment after introduction (2.0 ton-DS/day) than in conventional one (2.9 ton-DS/day). As for digester volume, the demonstrated equipment was smaller in size than the conventional one (HRT 20 days) in spite of the loading of garbage.

The introduction of the technology was estimated to reduce the construction cost and the maintenance cost by 25 and 38%, restrictively from that for the conventional one. Also, the greenhouse gas emissions were evaluated. The result obtained was that the reduction rate for GHG (LCCO₂) was 86% compared with conventional technology.

In line with an increase in raw sludge generation and a decrease in excess sludge generation, the load on the thickening process shifts from a more-energy-consuming mechanical thickener to a less-energy-consuming gravity thickener. At the same time, an increase in raw sludge with a greater digestion rate expands the sludge reduction effect.

(1) Intensive solid–liquid separation

An SS removal rate of 70% (raw water concentration, 145 mg/L) and a BOD removal rate of 47% (raw water concentration, 170 mg/L) could be achieved at a filtration rate of 250 m/day.

(2) Thermophilic digestion

At the rated load (HRT 5 days, VS load 6.0 kg/m³/day), the VS decomposition rate was 70% on average, and the digestion gas generation was 759 Nm³/t-loaded VS on average. Subsequently, continuous operation of more than 200 days could be done while changing the loading conditions.

(3) Smart power generation system

Electric power consumption could be reduced by 55% due to the effective utilization of the digestion gas and demand control. The generated digestion gas was totally used for the fuel cell. The generation efficiency was 40%.

(4) Case study

On the basis of the demonstration data, some effects of introduction were calculated for a sewage treatment plant of a scale of 50,000 m³/day. The construction cost, maintenance cost and GHG emission were reduced by 25%, 38%, and 86%, respectively.

The study showed that the introduction of the three elemental technologies described above can result in reduction for plants undertaking conventional mesophilic digestion.

The authors would like to express our gratitude for the cooperation of the Public Works Bureau and Environment Bureau of Osaka City.