The amount of biosolids production can be reduced by extending solids retention time (SRT). Biosolids reduction occurs by two different mechanisms in a typical activated sludge process.
- Microorganisms lose their mass slowly to generate energy to maintain the life, which is so called “endogenous respiration”. Active transport of ions against osmotic pressure through the cell wall, repairing damaged cell wall, producing enzymes, etc. are some examples of the activities microorganisms do consistently.
- The incessant circle of life in microbial community is other major contributor of biosolids reduction at high SRT. Dead cells are hydrolyzed and consumed by live cells to be born as new cells. The cell reproduction process is not 100% efficient and some biomass is lost as CO2.
The rate of mass reduction slows down with time because non-biodegradable portion of the microorganisms accumulate in the sludge. For example, the cell wall of microorganisms are designed to protect microorganisms from hydrolysis and the enzymatic attack from other microorganisms. According to the IWA’s activated sludge model number 1 (ASM#1), the non-biodegradable portion of microorganisms is assumed around 8% of the total biomass. Therefore, the 8% of non-biodegradable portion accumulates in the mixed liquor every time the microbial generation moves on.
The growth of microorganisms can be described as Eq.(1) neglecting the accumulation of non-biodegradable MLSS, where xl is MLSS, Se is COD, Ks is a half-reaction constant, um is the maximum growth rate constant, and kd is death rate. If there is no sludge removal, xl (MLSS) increases over time since the growth rate is higher than the death rate. However, if the death rate, kd, can be increased artificially, MLSS increasing rate can decrease. .
Apparent growth rate = Growth rate – Death rate
In order to increase kd, artificial sludge disintegration can be introduced. As shown in Fig. 1, mixed liquor from the aeration tank is disintegrated in a separate tank and recycled back to the aeration tank. The protoplasm, debris, etc. are consumed by the live microorganisms in the aeration tank and reproduced as new microorganisms. The amount of the new microorganisms is always less than the disintegrated microorganisms due to the loss in the reproduction process. By controlling the rate of sludge disintegration (q), desired biosolids reduction can be obtained within the limitation imposed by the accumulation of non-biodegradable matters. If the sludge disintegration method employed can convert non-biodegradable matters to biodegradable, the biosolids production can approach to near zero.
Many different ways of sludge disintegration have been developed since the first attempt of thermophilic digestion was published in 1980’s (Torpey, 1984). The most successful commercial method in spite of the high capital and operating costs might be ozonation (Yasui, 1996, Park, 2003). Some examples of commercial process using ozone include BioleaderTM(Kurita), LysoTM (Praxair), HaliaTM (Air Product), Aspal SLUDGETM System (Air Liquide), etc. Later physico-chemical methods such as ultrasound, ball mill, plasma, electrolysis, alkaline/acid, thermal hydrolysis, etc. were introduced.
Biological methods have been also introduced, where activated sludge is circulated through the side biological reactor with very different environment such as anoxic (CannibalTM, Siemens), anaerobic (Chon, 2010) ), thermophilic (S-TE, Kobelco Eco-solutions) conditions. A good portion of activated sludge die off in the side reactors due to the dramatic environment change and converted to the already adapted species. When the mixed liquor adopted in the side reactor is recycled back to the aeration tank, bacterial population again changes back to original. While the microbial population transforms back and forth repeatedly, biomass can reduce. However, there are some observations and reports that argues about the long-term efficacy of the biological methods (EPA 600/S2-90/037). Perhaps the microbial population is eventually dominated by a group of microorganisms that can grow in the two extreme conditions so that the biosolids reduction fades out over time, but little has been known about the inconsistent biosolids reduction performance of such processes. In addition to the chemical, physical, and biological methods, there exist hybrid processes such as AFC (PMC Biotech), Biothelys (Veolia), CrownTM(Biogest), thermochemical process (Banu, 2011)etc.
Fig. 1. Principle of biosolids reduction by partial sludge disintegration and the recycle to aeration tank (Yoon, 2003)
Factors affecting economics
As an indicator of the efficiency of sludge reduction process, sludge disintegration number (SDN) was introduced. It was defined as the ratio of the amount of treated to the amount reduced (Yoon, 2003). Based on the initial studies performed with ozone, SDN was approximately 3 when ozone dose was 0.05 g O3/g MLSS each time mixed liquor was treated (Yasui, 1996; Sakai, 1997). As a result, the specific ozone demand to reduce sludge was around 0.15 g O3/g MLSS removed. In the same study performed in an activated sludge plant with a clarifier, no excess biosolids was removed from the system for almost a year. This observation suggested that ozone converted non-biodegradable matters to biodegradable so that non-biodegradable matters did not accumulate indefinitely.
There are two major factors affecting the economics of biosolids reduction.
- Cost of sludge disintegration – The capital and operating costs of sludge disintegration process vary widely depending on the method of sludge disintegration. Biosolids reduction using biological methods such as anoxic, anaerobic, or themophilic reactors are less costly than chemical or physical methods, but according to some reports the efficacy is fuzzier than chemical and physical methods.
- Extent of target biosolids reduction – Cost of biosolids reduction increases disproportionally when the target biosolids reduction increases. For example, targeting 30-50% biosolids reduction is much more economical than targeting 70-90%. It is because sludge disintegration process becomes less efficient when complete disintegration (or solubilization) is targeted. In addition, the disintegrated sludge sent to aeration tank can come back to the disintegration process before biologically treated.
© Seong Hoon Yoon