“Inorganic + Organic Toxic Compounds” and “Nutrients + Macronutrients”.

“Inorganic + Organic Toxic Compounds” and “Nutrients + Macronutrients”.

Proper analysis and treatability studies are needed to assure that a chronic toxicity does not exist for wastewater treated by anaerobic processes. Athe the same time, the presence of a toxic substance does not mean the process cannot function. Some toxic compounds inhibit anaerobic methanogenic reaction rates, but with a high biomass inventory and low enough loading, the process can be sustained. Toxic and inhibitory inorganic and organic compounds of concern for anaerobic processes are presented in table A-1 and A-2, respectively.

Acclimatization to toxic concentrations has also been shown (Speece, 1996). Pre-treatment steps may be used to remove the toxic constituents, and, in some cases, phase separation can prevent toxicity problems by providing for degradation of the toxic constituents in the acid phase, before exposure of the more sensitive methanogenic bacteria to the toxic constituents (Lettinga and Hulshoff Pol, 1991).

Table A-1 Toxic and inhibitory inorganic compounds of concern for anaerobic processes^a

Substance	Moderately inhibitory concentration	Strongly inhibitory concentration
	mg/l	mg/l
Na+	3500 – 5500	8000
K+	2500 – 4500	12000
Ca2+	2500 – 4500	8000
Mg2+	1000 – 1500	3000
Ammonia-nitrogen NH4+	1500 – 3000	3000
Sulfide, S2-	200	200
Copper, Cu2+		0.5 (soluble) 50 – 70 (total)
Chromium, Cr(VI)		3.0 (soluble) 200 – 250 (total)
Chromium, Cr(III)		2.0 (soluble) 180 – 240 (total)
Nickel, Ni2+		30.0 (total)
Zinc, Zn2+		1.0 (soluble)

^aForm Parkin and Owen (1986).

Toxic and inhibitory organic compounds of concern for anaerobic processa

Compound	Concentration resulting in 50 percent reduction in activity
	mMb
1-chloropropene Nitrobenzene Acrolein 1-chloropropane Formaldehyde Lauric acid Ethyl benzene Acrylonitrile 3-chlorol-1,2-propanediol Crotonaldehyde 2-chrolopropionic acid Vinyl acetate Acetaldehyde Ethyl acetate Acrylic acid Catehol Phenol Aniline Resorcinol Propanol	0.1 0.1 0.2 1.9 2.4 2.6 3.2 4 6 6.5 8 8 10 11 12 24 26 26 29 90

^aFrom Parkin and Owen (1986).
^bmM = millimole.

Nutrients

Though anaerobic process produce less sludge and thus require less nitrogen and phosphorous for biomass growth, many industrial wastewaters may lack sufficient nutrients. Thus, the addition of nitrogen and/or phosphorous may be needed. Depending on the characteristics ofthe substrate and the SRT value, typical nutrient requirements for nitrogen, phosphorous, and sulfur are in the range from 10 to 13, 2 to 2.6, and 1 to 2 mg per 100 mg of biomass, respectively. The values for nitrogen and phosphorous are consistent with the values for these constituents estimated on the basis of the composition of the cell biomass. Further, to maintain maximum methanogenic activity, liquid phase concentration of nitrogen, phosphorous, and sulfur on the order of 50, 10 and 5 mg/l, respectively are desirable (Speece, 1996).

Macronutrients

The importance of trace metals to stimulate methanogenic activity has been noted and discussed by speece (1996). The recommended requirements for iron, cobalt, nickel, and zinc are 0.02,0.004, 0.003, and 0.02 mg/g acetate produced, respectively. Examples of increased anaerobic activity were noted after trace additions of iron, nickel, or cobalt. The exact amounts of trace nutrients needed can vary for different wastewaters, and thus trial approaches are used to assess their benefit for anaerobic process with high VFA concentrations, a recommended dose of trace metals per liter of reactor volume is 1.0 mg FeCl₂, 0.1 mg CoCl₂, 0.1 mg NiCl₂, and 0.2 ZnCl₂.