Digestion CE 431. First, some basic questions: o Where do the solids come from? o Why do we need to perform additional treatment?

Transcription

1 First, some basic questions: o Where do the solids come from? Digestion CE 431 o Why do we need to perform additional treatment? Lecture Notes - Digestion.doc 1

2 o How are they regulated? See also handout on Washington regs EPA Part 503 regulations Class A = best quality Fecal coliform < 1000 MPN/gram total solids Vectors evaluated separately from pathogens o EPA published 10 vector reduction alts (see handout) o e.g., achieve 38% reduction Trace elements - simply comply with maximum concentrations Can sell as fertilizer Can be land applied with few reporting or management requirements Class B = lower quality (easier to achieve treatment req ts) Fecal coliform < 2,000,000 MPN/gram total solids Same vector requirements as Class A o Often target barrier process - injection or tillage Same trace elements as Class A Typically land applied on ag crops o How are they typically treated? Lecture Notes - Digestion.doc 2

3 Anaerobic digestion: o Most common biosolids treatment technique o General definition of anaerobic process: A biological process that produces energy in the absence of molecular oxygen o HIGHLY complex microbial consortia that ultimately yields methane gas as the principle end product o Three principle biomechanistic processes: Lecture Notes - Digestion.doc 3

4 o Qualitative summary of mechanistic steps: Hydrolysis Acid formation Methane formation Lecture Notes - Digestion.doc 4

5 o Discussion on process mechanistics and influencing factors CO 2 and alkalinity MCRT and HRT Needs to be sufficient to accomplish VSS reduction Systems without recycle, MCRT=HRT Changing MCRT results in changes in each of the three mechanistic processes There is a minimum MCRT for each step - if too short for a given step, bacteria will die and process will fail Based on a variety of factors, including temp, waste characteristics, ph Overall process not well defined for modeling purposes Kinetics and stoichiometry Lecture Notes - Digestion.doc 5

6 Anaerobic Digestion - Design o Fully enclosed tanks - maintain anaerobic conditions; control odors o Process fundamentals: SRT, HRT 3 principle rxns directly a f(srt) There is a minimum SRT for each microbial group Temperature Controls microbial growth rates Thus controls SRT Controls which microbial species are present Mesophilic = F Thermophilic = F o Faster reactions - why? Estimate on how much faster? Methanogens are particularly sensitive to T changes How controlled? Alkalinity Again, CO 2 consumes But why should we care? Provide mixing? Lecture Notes - Digestion.doc 6

7 o Low-rate versus High-rate Digesters - main differences Organic loading SRT Degree of mixing Temperature o Low-rate (aka standard-rate) not really used anymore Require too much tankage Inadequate mixing o Separate the solids streams? Why? o We will specifically focus on Mesophilic single-stage, high-rate digestion Historically (and to a certain degree still) the microbiology and biochemistry were not well understood, thus the process design is based mostly on empirical models (sound like another bio-treatment process?) SRT=HRT, since no recycle Typical SRT min - see table in your book Typical stoichiometric and kinetic relationships Lecture Notes - Digestion.doc 7