The power of poo

Ef­fi­ciency drives, waste min­imi­sa­tion and re­duc­ing green­house gas emis­sions + ris­ing oil prices = power from the strangest places. El­e­ment holds its nose and in­ves­ti­gates how sewage and other waste could off­set the equiv­a­lent of more than the elec­tric­ity

Element - - Primary Industry - By Andy Ken­wor­thy

We all make bio­gas, but we usu­ally blame the dog. It comes from any de­com­pos­ing wet or­ganic waste. It is mainly meth­ane and car­bon diox­ide, as well as very small amounts of hy­dro­gen sul­phide and other im­pu­ri­ties.

The meth­ane is more of a worry than the smell. If re­leased into the at­mos­phere it has a 20 times greater green­house ef­fect than car­bon diox­ide. But bio­gas can be col­lected to cre­ate nat­u­ral gas-type fu­els for homes and ve­hi­cles. When used as a fuel the meth­ane is con­verted into the less harm­ful car­bon diox­ide. It’s a win-win sit­u­a­tion: we get fuel and we re­duce the po­ten­tial for cli­mate change. But it’s not without its lim­i­ta­tions.

Waste­water and sewage treat­ment plants gen­er­ate bio­gas as part of nor­mal op­er­a­tion, so they are a great place to har­ness it. The lat­est pi­lot plant do­ing this in New Zealand is Ro­torua District Coun­cil’s Waste­water Treat­ment Plant, and is part of the Crown Re­search In­sti­tute Scion’s Waste 2 Gold pro­gramme. As well as har­ness­ing the bio­gas for fuel, it uses the re­sult­ing heat to break the solid waste down into re-use­able chem­i­cals and a range of other by-prod­ucts. Some of these can even be used for fer­tilis­ers or in the pro­duc­tion of bio­plas­tics.

The Ro­torua pi­lot is ini­tially only set to run for 12 months. If it works, the next stage would be a full-scale fa­cil­ity di­vert­ing as much as pos­si­ble of the 8,500 tonnes of biosolid waste – sewage, food scraps – from the cur­rent treat­ment plant. The project team be­lieves sav­ings from con­vert­ing the waste to en­ergy, other reduced plant op­er­at­ing costs and in­come from selling the new prod­ucts, could cre­ate to­tal ben­e­fits for the coun­cil and com­mu­nity of about $4 mil­lion a year. Not bad for a pile of…

This in­cludes not hav­ing to spend $900,000 on bury­ing solid sewage waste in land­fill. How­ever, the land­fills also have a pos­i­tive as the or­ganic waste buried there also de­com­poses and pro­duces bio­gas. Coun­cils in Hamil­ton, Christchurch, Dunedin, North Shore, Palmer­ston North and Porirua all op­er­ate bio­gas har­ness­ing sys­tems at land­fill sites. For ex­am­ple, Mighty River Power op­er­ate four in New Zealand: Green­mount and Rosedale, which to­gether pro­duce 8.2 megawatts of elec­tric­ity, and Tiro­hia and Sil­ver­stream, which pro­duce two megawatts each.

Pro­duc­ing bio­gas from waste pro­duced at in­dus­trial food pro­cess­ing plants is an­other pos­si­bil­ity. You can even grow crops, in­clud­ing most an­i­mal fod­der as well as sorghum, specif­i­cally to process into fu­els in this way, although this is not yet done in New Zealand.

Cur­rently, bio­gas sys­tems within New Zealand only pro­duce a neg­li­gi­ble amount of elec­tric­ity. How­ever it is most valu­able when used to supplement or off­set the heat or elec­tric­ity needs of the host plants in which it is made.

Brian Cox, from the Bioen­ergy As­so­ci­a­tion of New Zealand, reck­ons the in­dus­try will con­tinue to develop along these lines, as a smart way to get value from waste, rather than a key en­ergy provider to the na­tion. He also ar­gues that “Bio­gas’s best value will be in be­ing used as a ve­hi­cle fuel, but this is only re­ally ef­fi­cient when it is used at source, for ex­am­ple, by the trucks op­er­at­ing at or from a land­fill that has a bio­gas di­gester. Any­thing like a na­tional dis­tri­bu­tion net­work for bio­gas would be too costly.

Even so, the as­so­ci­a­tion es­ti­mates that New Zealand could be pro­duc­ing up to 15 peta­joules of en­ergy from bio­gas per year by 2040, or even dou­ble that if ris­ing oil prices or sup­ply prob­lems stim­u­late the grow­ing of “fuel crops” to con­vert to bio­gas.

This lower fig­ure is equiv­a­lent to one and a quar­ter times the amount of elec­tric­ity used in the greater Christchurch area and would help re­duce our re­liance on im­ported oil or the temp­ta­tion to mine the South Is­land’s lig­nite coal to con­vert to diesel.

The Water­care waste­water treat­ment plant, Man­gere.

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