Sewage treatment carbon neutralization operation requires sludge increment

The progress of sewage treatment towards carbon neutral operation has already become the future development direction of sewage treatment in Europe and the United States. If the carbon neutral operation is targeted, the excess sludge will obviously no longer be the “burden” of sewage treatment, and will become a hot material for carbon neutral operation. To this end, the sewage treatment industry no longer needs to blindly pursue sludge reduction, and instead expects sludge to increase, in order to increase the self-sufficient raw material share of sewage treatment energy.

However, the amount of excess sludge depends entirely on the level of influent COD load. A low COD load will inevitably lead to less excess sludge production, which means that there may be a deficit in carbon neutral operating energy demand. Therefore, the increase of sludge targeting carbon neutralization has quietly emerged in the world in recent years. In addition to conventional excess sludge, it will seek sludge increments from other sources, such as front-end screens. Divided into COD technology, back-end anaerobic co-digestion technology.

Status of foreign surplus sludge energy conversion

Sewage treatment carbon neutralization operation to achieve energy self-sufficiency in the entire sewage treatment process, in order to achieve this goal, Europe and the United States, and even some surrounding Asian countries have successively issued a road map for the 21st century sewage treatment carbon neutralization operation, and put into practice. E.g:

As early as 2008, the Netherlands proposed the concept of wastewater treatment NEWs, describing the future sewage treatment plant as “Nuttrient”, “Energy” and “Renewable Water” (Factories). Operating mode.

At present, some sewage treatment plants in Europe and the United States use excess sludge as the main energy carrier, combined with front-end screening COD (influent COD load high) technology, or back-end anaerobic co-digestion (food waste, food processing waste, feces) Technology) to obtain energy (CH4) from sewage or exogenous organic matter by anaerobic digestion in a manner that maximizes “sludge increment” and has fully or partially achieved carbon neutrality operational targets.

Sludge energy conversion carbon neutralization potential

Some sewage treatment plants that implement carbon neutrality operation targets in countries such as Europe and the United States are mostly based on anaerobic digestion and conversion of energy from surplus sludge. Countries such as Europe often have higher influent COD concentrations (600~1 000 mg/L) due to lifestyle habits, no septic tanks, rain and sewage diversion, and food breakage. Some examples of wastewater treatment in Europe and the United States with carbon neutrality indicate that if the influent COD is 600 mg/L, the amount of excess sludge produced by conventional treatment processes (such as A2/O denitrification and dephosphorization process) is passed. Anaerobic digestion and conversion energy is difficult to fully meet (100%) carbon neutral operation targets, generally achieving 70% carbon neutralization rate is sufficient.

The content of organic matter in sewage in China is much lower than that in Europe and the United States, so it is difficult to achieve the carbon neutralization target only by the surplus sludge generated. Figure 1 plots the COD rejection of the excess sludge (primary and secondary sedimentation) in the energy balance calculation (the ratio of total COD to influent COD in the sludge) versus carbon neutralization. The trend in Figure 1 shows that more sludge neutralization is required to achieve greater carbon neutrality, ie the so-called “sludge incremental” concept. From the perspective of the source of COD, the increase of sludge means that the COD in the influent should avoid the direct oxidation of COD without purpose, in addition to the demand for carbon source for nitrogen and phosphorus removal.

Sludge increment method and measure

A/B method A section concentrated COD

As early as 15 years ago, in collaboration with Professor Mark van Loosdrecht of TU Delft, a sustainable municipal wastewater treatment centered on energy and phosphorus recovery, we proposed the conceptual process shown in Figure 2. . In order to effectively intercept the excess COD (except the carbon source required for nitrogen and phosphorus removal) and convert it to methane by anaerobic digestion, the A section of the German A/B method was used to concentrate the suspended and dissolved COD. Compared with the second sedimentation sludge, the intercepted sludge in section A has better digestibility and can produce biogas with higher methane content.

Front-end screening COD technology

In order to maximize the retention of COD in the influent, European scholars have also proposed to use the microfiltration method after flocculation to intercept the colloidal and dissolved COD, and use it for anaerobic digestion to convert methane into action. For example, a water company in Berlin, Germany, and the German KWB organization have launched an application research project for the recovery of energy in wastewater – CARRISMO (carbon is money), the process flow is shown in Figure 3.

Sludge co-digestion technology

The co-digestion of sludge exerts a synergistic effect between the substrates, which improves the degradation rate and degradation degree of the substrate, and the energy conversion efficiency is significantly improved. Table 1 lists the energy conversion effects of several different exogenous organic wastes and residual sludge, and the co-digestion potential of excess sludge and other organic wastes is evident.

Table 1 Different types/proportions of exogenous matrix and sludge co-digestion energy conversion effect

From a technical point of view, there is no obstacle to the carbon neutralization operation of sewage treatment, which is mainly limited by the government's macro environmental policy. As long as the government is far-sighted, policy support and even financial subsidies will touch the sewage treatment industry toward carbon neutrality, thus gaining comprehensive environmental benefits that are generally optimistic.

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