Question 3 of 5: Suspended-solids test and the high-rate anaerobic digester

Question 3: Suspended-solids test and the high-rate anaerobic digester (25 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a)(i) Total, volatile and fixed suspended solids (7 marks)

Given. A gravimetric suspended-solids determination on a sludge sample, with the four weighings and the sample volume listed below.

Find. The total suspended solids, the volatile suspended solids and the fixed suspended solids, each expressed in mg/L.

Approach. Total suspended solids are the mass gained by the filter on drying, divided by the volume filtered; ignition at 550 °C burns off the organic fraction and the paper itself, so what remains is the fixed (inorganic) residue; volatile solids are the difference.

1. Total suspended solids from the mass gained on drying. The solids retained on the filter are the difference between the two weighings, and the concentration is that mass divided by the volume filtered: $$\mathrm{TSS}=\frac{m_2-m_1}{V} =\frac{1.2345-1.1234\ \text{g}}{0.025\ \text{L}} =\frac{0.1111\ \text{g}}{0.025\ \text{L}}=4.444\ \text{g/L}$$ Converting to the units the question asks for gives $$\boxed{\mathrm{TSS}=4444\ \text{mg/L}}$$
2. Fixed suspended solids from the ignition residue. Ignition at 550 °C oxidises the organic matter (and the cellulose filter) to carbon dioxide and water, leaving the inorganic ash: $$\mathrm{FSS}=\frac{m_3}{V}=\frac{0.0234\ \text{g}}{0.025\ \text{L}} =0.936\ \text{g/L}=\boxed{936\ \text{mg/L}}$$
3. Volatile suspended solids by difference. The volatile fraction is what was lost on ignition, so it is simply the total less the fixed residue: $$\mathrm{VSS}=\mathrm{TSS}-\mathrm{FSS}=4444-936=\boxed{3508\ \text{mg/L}}$$
4. Interpret the ratio. Expressing the volatile solids as a fraction of the total, $$\frac{\mathrm{VSS}}{\mathrm{TSS}}=\frac{3508}{4444}=0.789\equiv 79\ \%$$ A raw primary or waste-activated sludge normally runs 70–80 per cent volatile, so the sample is consistent with an undigested municipal sludge; a well-digested sludge would have fallen to roughly 50 per cent.

(a)(ii) Effect of igniting at 900 °C instead of 550 °C (3 marks)

Standard Methods specifies 550 ± 50 °C precisely because that temperature is hot enough to burn organic matter completely but not hot enough to decompose the mineral fraction. At 900 °C several additional losses occur and all of them bias the result the same way. Carbonates decompose, principally

with magnesium carbonate going even earlier, so 44 g of carbon dioxide is driven off per 100 g of calcium carbonate present. Magnesium hydroxide and other hydroxides dehydrate; ammonium salts, and chlorides such as magnesium and calcium chloride, volatilise; and some of the crystallisation water bound in the mineral matrix is expelled. The consequence is that the ignition residue is too light, so the fixed suspended solids are underestimated, and because volatile solids are obtained by difference the volatile suspended solids are correspondingly overestimated. Total suspended solids, which are measured before ignition, are unaffected. A chemically precipitated or lime-conditioned sludge, rich in calcium carbonate, could show an apparent volatile fraction inflated by ten percentage points or more, which would in turn make a digester look better fed and less stabilised than it really is. There is also a practical consequence: 900 °C exceeds the rated service temperature of ordinary porcelain crucibles and glass-fibre filter discs, so the test may have to be repeated in any case. The correct response is to discard the result and repeat the ignition on a fresh, correctly dried sample at 550 °C.

(b) Principle and working of a high-rate anaerobic digester (15 marks)

A high-rate digester is a heated, continuously fed and completely mixed anaerobic reactor in which sewage sludge is stabilised in the absence of oxygen. It is distinguished from the older standard-rate digester by three deliberate design choices: the contents are mixed, so the whole tank volume is active rather than stratifying into scum, supernatant, digesting and settled zones; the contents are heated to a controlled mesophilic temperature of about 35 °C; and the feed is thickened and applied continuously or in frequent small doses rather than intermittently. Those three choices cut the required retention time from 30–60 days to 15–20 days for the same volatile-solids destruction, and correspondingly cut the tank volume, which is where the economics of the process live.

Principle — the four-stage anaerobic food chain. Stabilisation proceeds through a sequence of microbial groups that must remain in balance. In hydrolysis, extracellular enzymes break particulate proteins, carbohydrates and lipids into soluble amino acids, sugars and fatty acids; this is the rate-limiting step for a particulate feed such as sewage sludge. In acidogenesis, fermentative bacteria convert those monomers into volatile fatty acids, alcohols, hydrogen and carbon dioxide. In acetogenesis, the longer-chain acids are oxidised to acetate, hydrogen and carbon dioxide, a reaction that is only thermodynamically favourable if the hydrogen partial pressure is kept very low. Finally, in methanogenesis, two distinct archaeal groups produce methane, the acetoclastic route

which accounts for roughly 70 per cent of the gas, and the hydrogenotrophic route

which consumes the hydrogen and so keeps the acetogens running. The methanogens are the slow, sensitive link in the chain: they grow with a minimum doubling time of the order of days, are inhibited below about pH 6.6, and are poisoned by free ammonia, sulphide and heavy metals. Every feature of the reactor design exists to protect them.

Working. Thickened raw sludge at 4–6 per cent total solids is fed continuously to the mixed volume, and an equal volume of digested sludge is withdrawn from the conical floor, so that the solids retention time equals the hydraulic retention time — there is no biomass recycle, which is why the retention time cannot be reduced below the methanogens' washout limit of roughly ten days without risking souring. Mixing is by recirculated digester gas injected through lances or a draft tube, by mechanical draft-tube impellers, or by external pumped recirculation; it must turn the contents over every 20 to 30 minutes to keep the solids in suspension, prevent a scum blanket forming under the cover, and distribute the incoming feed so that no pocket of the tank sees a local overload of volatile acids. Heat is added by passing recirculated sludge through an external spiral or tube-in-tube heat exchanger fed with hot water from the biogas boiler or engine jacket; the heat load is the sum of raising the incoming sludge from ambient (near 5 °C in a Canadian winter) to 35 °C plus the tank's fabric losses, and holding the temperature steady to within about ±0.5 °C per day matters more than the absolute value, because methanogens tolerate a temperature far better than they tolerate a change in it.

The gas produced collects under a floating or membrane cover that also provides short-term storage and maintains a slight positive pressure so that no air can be drawn into an explosive mixture. Typical performance for a 15-day mesophilic digestion of a mixed primary and waste-activated sludge is 45–55 per cent destruction of volatile solids, yielding roughly 0.9–1.1 m³ of biogas per kilogram of volatile solids destroyed at 60–70 per cent methane, i.e. an energy content of about 22–26 MJ/m³. That gas ordinarily covers the digester's own heating demand and a useful share of the plant's electrical load through a combined heat and power unit. Because the contents are mixed there is no supernatant layer to decant — the whole withdrawal goes to dewatering — and the digested product is a stabilised, largely deodorised biosolid with a much reduced pathogen count and improved dewaterability. Routine control is by monitoring the volatile acid to alkalinity ratio (kept below about 0.3 by adding alkalinity if necessary), gas production rate and methane fraction, pH, and temperature; a falling methane fraction with rising volatile acids is the classic early signature of organic overload, and the response is to cut the feed rate rather than to add more.