GENERAL:
The primary sedimentation sludge and usually the scum, along with the secondary treatment process waste solids, such as "waste activated sludge" or "fixed-film media microbes" produced by the secondary process units are usually thickened and pumped to a digester. Most often, it is heated in an anaerobic (no oxygen) state at a temperature around 95 degrees Fahrenheit, in a "mesophilic anaerobic digester" (the most popular). This reduces and stabilizes the solids and microbes further. "Thermophilic" (rarely constructed due to the much higher energy requirements to heat, and more difficult to operate properly) digesters operate well at temperatures between 110 and 140 degrees F. After approximately three weeks optimum, (16 days minimum), the solids are ready to pass through to the solids dewatering process. Factors affecting the time required for stabilization of the solids include the origin/type of solids, digester temperature, and pH.

ANAEROBIC DIGESTER TEMPERATURE PARAMETERS:
Temperature ranges: psychrophilic less than 80 deg F
mesophilic 80 to 110 deg F
optimum 95 deg F
thermophilic more than 110 deg F

Do not change the temperature in the digester more than 1 degree per day to "culture and pamper" the methane producers. There are many different species of methane producers, each with it’s own required temperature range, and as such, changing/fluctuating temperatures does not allow for the maximizing of a set of species to accomplish the conversions.

MESOPHILIC ANAEROBIC DIGESTER PARAMETERS:
Digester gas:
Composition methane 65-75%
CO2 30-35%
other 1%
Quantity
12 to 18 ft3/lb of VS destroyed: (this is about 1.0 ft3/day/person for a secondary treatment plant)

Heat value 500-1,000 BTU/cuft (usually about 950)

Volatile solids reduction: 40 -60% range

pH desired range of 6.8 to 7.2

Detention Times: 15 to 20 days for a well mixed "high rate" digester

Alkalinity: 1,000 to 5,000 mg/L with Volatile Acids < 500 mg/L
Volatile Fatty Acids < 250 mg/L

In summary: (PART II will cover the microbes and the chemical reactions next month.)
Acid formers: convert complex compounds into organic fatty acids, ammonia, carbon dioxide, and alcohol's. They are hearty, can live in acidic environment, are easy to grow, and are like the "carp" of the bacteria culture.

Methane producers: convert hydrogen and carbon dioxide to methane; and acetate to methane and bicarbonate. Methane producers are very frail and impacted easily by changes and effects of pH, toxins, temperature. They are the delicate "trout" of the bacteria culture. As operators we culture and pamper these methane producers.

Volatile Solids: generally speaking, those solids that can be converted into a gas by heating in an electric muffle furnace (usually 550 deg C for 60 minutes); the ash that remains in the dish is usually described as "non-volatile inorganic solids"

Volatile Solids Loading Rate. Prior to reviewing the methods in which the volatile solids loading rates may be calculated, we should discuss and possibly agree upon several assumptions:
1) Many loading rate calculations such as these, are all empirically derived. That is, in the past we designed and built several similar processes. We later found in operating these process units that some performed better than others. We then created ways in which to compare and quantify their operation, in what we call "operational parameters" so that we could designed, build, and operate more effective processes, based on those operating well.

2) Once we have described an operational parameter, such as pounds VS/cubic foot or a detention time, we then have to determine the best range of values for it. Finally, as operators we then do our best to adjust all of the variables to maintain these operational parameter ranges to optimize the treatment process.

3) If we agree on #1 and #2 above, then it is not unreasonable to expect that we can create new operational parameters, test them in actual operation and process control, and then incorporate those that are of value in our operational strategies. For the "more experienced operators" (translated means "older gals and guys") among us: remember when we used the "sludge age" formula for operating our activated sludge plants, and now most of us use the "mean cell resident time" formula in its place?

4) Anaerobic digester process control is primarily by a) hydraulic/solids feed rate, in pumping schedule with detention time, & b) temperature control. Temperature and detention time are critical elements where a stable digester temperature with frequent, "small feedings of raw sludge" or a very low continuous feed rate is best. Longer detention time values allow the creation of a "buffering" alkalinity to form and help stabilize the microbial community’s environment. Organic loading rates are recognized, but are secondary in nature, as an "after-the-fact" calculation. The ideal operational strategy is a continuous pumping of a concentrated raw sludge into a digester with consistent temperature control and adequate attention time. The detention time is critical, as the methane formers are slower to grow, more sensitive to changes, and with a short detention time are washed out of the digester. Then as an "after-the-fact" calculation, we determine the VS loading rate as a check of the solids concentration and/or liquid sludge rate, into the digester.

5) When the raw sludge feed concentration is about < 3% Total Solids (TS), the corresponding "light" VS solids loading rate is of no consequence. The detention time controls the process due to the dilute sludge causing hydraulic detention time limitations. On the other hand, when the raw sludge feed concentration is > 3%, the heavier organic loading may now impact the volatile solids loading rate in the available detention time period. At the extreme, pre-thickening may be too effective, resulting in a thick sludge in the digester that is also too hard to mix effectively. (For feed sludges greater than 6% to 8% with short detention times, the digester may have a difficult time reducing/stabilizing the solids resulting in a thick, hard to mix sludge in the digester.) I wish to add: very low volatile solids loading rates with correspondingly high hydraulic loading rates (short detention times), may suggest that the sludge thickening prior to feeding the digester would be more cost-effective than building more digester capacity. We typically feed a 5% total solids concentration and the digester contents are 2.1% total solids concentration.

Being a "wise guy utilities operations manager" we can convert each of the above formulas into metric units! We will start with our favorite: kg VS/day/m3 and move through them all! OK maybe not!

PART II will cover the microbes and the chemical reactions.

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